- Email: [email protected]
Triplet state of 4-biphenylcarbonyl and 2-naphthylcarbonyl chromophores in homopolymers, copolymers and model compounds
Polymer Photochemistry 3 (1983) 119-129
Triplet State of 4-Biphenylcarbonyl and 2-Naphthylcarbonyl Chromophores in Homopolymers, Copolymers and Model Compounds Pavol Hrdlovict, Ghislain Guyot,$ Jacques Lemaire$ and Ivan Lukact t Polymer Institute of the Slovak Academy of Sciences, CS-842 36 Bratislava, Dubravska cesta, Czechoslovakia ~:Laboratoire de Photochimie Mol6culaire et Macromol6culaire (6quipe associ6e au CNRS no. 929), Universit6 de Clermont II, B.P. 45, 63170 Aubiere, France (Received: 22 March, 1982)
ABSTRACT The triplet state of poly(1-(4-biphenyl)-2-propen-l-onc), poly(1-(2naphthyl)-2-propen-l-one), copolymers of the above carbonyl monomers with methyl methacrylate and respective model compounds was sought in solution at room temperature by emission spectroscopy and microsecond ]lash photolysis. Although 4-biphenylcarbonyl and 2-naphthylcarbonyl yield an intense phosphorescence at 77 K, no emission was observed in solution at room temperature. The intense transient spectrum observed in benzene solution for 1-( 4-biphenyl)-3chloropropan-l-one and 1-(2-naphthyl)-3-chloropropan-l-one was ascribed to the triplet state. Self-quenching was found to decrease the intensity of the transient spectrum and the life-time when building up both chromophores in a macromolecule. More e]~cient self-quenching was observed for 2-naphthylcarbonyl than for the 4-biphenylcarbonyl chromophore which was suggested as a suitable triplet probe. The limiting triplet life-time of 4-biphenylcarbonyl built up in a copolymer with methyl methacrylate at low concentration in a thermodynamically good solvent is about 0-3 ms. INTRODUCTION T h e r e l e v a n t d a t a of t he p h o t o p h y s i c a l and p h o t o c h e m i c a l processes of p o l y ( 1 - ( 4 - s u b s t i t u t e d p h e n y l ) - 2 - p r o p e n - l - o n e ) s , c o p o l y m e r s of t he 119
Polymer Photochemistry 0144-2880/83/0003-0119/$3.00 © Applied Science Publishers Ltd, England, 1983. Printed in Northern Ireland
120
Pavol Hrdlovic, Ghislain Guyot, Jacques Lemaire, Ivan Lukac
respective carbonyl m o n o m e r s with styrene and methyl methacrylate and of the model compounds have been recently reviewed. 1 It follows from this review that the electron-donating substituents in position 4 shift the low temperature emission bathochromically with disappearance of vibrational structure, prolong the life-time and strongly decrease the photochemical reactivity of the Norrish type II reaction in solution at room temperature. L2 This conclusion is supported by the laser flash photolysis measurements which showed that the lifetime for poly(1-(4-methoxyphenyl)-2-propanol-one), i.e. 800 n s , 3 is substantially longer than that for the unsubstituted homopolymer, i.e. 7-70 n s . 4"5 In this paper, we describe our attempts to characterise the triplet state of homopolymers, copolymers and model compounds of this type with 4-diphenylcarbonyl and 2-naphthylcarbonyl chromophores at room temperature in solution using emission spectroscopy and microsecond flash photolysis.
EXPERIMENTAL Model compounds, homopolymers and copolymers of carbonyl m o n o m e r s with methyl methacrylate are described in a previous paper, z Copolymer poly(1-(4-biphenyl) 2-propen-l-one-comethylmethacrylate) with 4 mass/mass % of carbonyl m o n o m e r was prepared in the same way as in Reference 2. H o m o p o l y m e r poly(1(4-biphenyl)-2-propen-l-one) was prepared by anionic polymerisation as described in Reference 6. B e n z o p h e n o n e (analytical reagent, Suchard), acetophenone, propiophenone, 4-methoxyacetophenone (purest grade, Fluka AG) were used without further purification. Emission spectra were taken on a Perkin Elmer M F P - 3 A instrum e n t from degassed carbon tetrachloride solutions (spectral grade, Merck). The solutions were degassed by seven-fold freeze-thaw cycles under high vacuum. The transient spectra and the decays were measured on a flash photolysis apparatus FPX-1 (Nortech Laboratories Ltd, England) in microsecond range. The photomultiplier output was fed to a Transient Recorder D L 905 (Data Laboratories Ltd) and recorded either on an oscilloscope Telequipment D 61a or on a recorder.
4-Biphenylcarbonyl and 2-naphthylcarbonyl chromophores
121
RESULTS A N D DISCUSSION Emission measurements In the previous p a p e r s 1'2 it was shown that poly(1-(4-substituted phenyl)-2-propen-l-one)s and their respective model compounds yield intense phosphorescence at 77 K in film or solid solution. Usually the n-~r* triplet is short-lived but the phosphorescence coming from the 1r-lr* triplet lives more than 100 ms. 1-3 In the experimental s e t - u p , 7 it was easy to observe the phosphorescence arising from the n - w * triplet level for benzophenone, propiophenone and acetophenone in solution at room temperature as in Reference 8. The introduction of 4-methoxy group in the acetophenone type molecule leads to a decrease in the quantum emission yield. At the highest sensitivity, no emission was observed for 4-methoxyacetophenone, 1-(4-methoxyphenyl)-3-chloro-1propanone, 1-(4-biphenyl)-3-chloro- 1-propanone and 1-(2naphthyl)-3-chloro-l-propanone. This shows that the radiationless transition of the triplet state depends on the temperature in a different way for n-~r* and ~--1r* levels. The emission of the lowest triplet state can be observed even at room temperature, when described as an n-Tr* type configuration. The emission of the lowest 7r-Tr* triplet state vanishes when the temperature increases from 77 K to 298 K. Emission measurements yield no information concerning the triplet states of chromophores under investigation. Flash photolysis measurements Flash photolysis in the microsecond region revealed that model compounds of 4-biphenylcarbonyl and 2-napthylcarbonyl chromophores yield a transient with a maximum around 400-425 nm. The relevant spectra and kinetic data of the transient spectra are summarised in Table 1. The intensity of the flash is proportional to the energy of the system U = ½CV2 where C is the charging capacity of the system and V is the voltage. The linear correlation between the flash intensity and concentration of the excited species indicates the involvement of a monophotonic process followed by a first-order decay, as the maximal intensity of the signal is considered as the initial intensity.
122
P a v o l Hrdlovic, G h i s l a i n G u y o t , J a c q u e s L e m a i r e , I v a n L u k a c
'i'
7
X
X ¢~
X
II
II
II
0 0
~g e~
<
i
?
9
0
O O 0 0 0 ~
9
I
z,
,
?
~
¢.J
-.z, ~.z, ~ .._, ~ v
44444N
I
nJ
8.~8~8
4-Biphenylcarbonyl and 2-naphthylcarbonyl chromophores
o
.=.
i
d~
~8
d~
?"
~'
~B 0
o
o
~ ??7?
z,~z
o
o
o
~Z~ a ~',.~
&~&
&
,~e,~
.o
&
.£ -
.-.
'rg
•o
°,o'~
,1= o
o
o
123
124
Pavol Hrdiovic, Ghislain Guyot, Jacques Lemaire, Ivan Lukac
0,3
AA
0,1
2
'
V2
'
26o
Fig. 1. The dependence of the absorbance at 425 nm of poly(1-(4-biphenyl)-2propen-l-one-co-methyl methacrylate) at 1 0 - 4 m o l e d m -3 on the intensity (1) in benzene; (2) in a mixture of benzene-isopropanol (2 : 3).
Figure 1 shows an excellent correlation of this type (line 1). If self-quenching takes place, there is a strong deviation from linearity at higher flash intensities (line 2). The decay is, in this case, still of pseudo-first-order because the n u m b e r of excited states seems to be lower than the n u m b e r of unexcited molecules. Since no p h o s p h o r e s c e n c e was observed from the above comp o u n d s at r o o m t e m p e r a t u r e in solution, it is necessary to identify the observed transient as a triplet or a radical. T h e following observations indicate that p r o b a b l y the lowest long-lived triplet is involved. 1.
2.
The life-time of the transient and the maximal intensity of the signal decreases as the concentration of the substrate increases in the range 1 0 - 4 - 1 0 -3 mole dm -3. This does not exclude a radical as a possible intermediate b u t the decay is always of the pseudo-first-order, which more satisfies a triplet self-quenching than a bimolecular decay for a radical. A n excess of a solvent with an easily abstractable hydrogen, i.e. isopropanol, influences only slightly the transient spectrum
4-Biphenylcarbonyl and 2-naphthylcarbonyl chromophores
125
of the model c o m p o u n d . Consequently, the formation of a radical does not occur. 3. The transient signal is q u e n c h e d by the addition of 4acetyloxy-2,2,6,6,-tetramethylpiperidine-N-oxyl which, however, is not consumed, as indicated by E S R measurements. Consequently, this radical acts as a triplet quencher. T h e quenching rate constants were calculated from the measurements at a single concentration. A t higher quencher concentration, the quenching rate constants were lower. T h e important r o u t e for the deactivation of the triplet state of the c h r o m o p h o r e s under study is self-quenching. Building up of the 4-biphenylcarbonyl c h r o m o p h o r e in a macromolecule leads either to an increased self-quenching for h o m o p o l y m e r or to a full exclusion of the self-quenching for c o p o l y m e r with methyl methacrylate with a low content of the a b o v e c h r o m o p h o r e (Fig. 2). Assuming the triplet life-time for c o p o l y m e r p o l y ( 1 - ( 4 - b i p h e n y l ) - 2 - p r o p e n - l - o n e - c o methyl methacrylate) to be 0-286 ms, then the rate constant of self-quenching k,q can be calculated, for the concentration 10 -3 and 10 -4 mole d m -3, respectively 5 - 8 4 × 106 d m 3 m o l e -1 s -1 and 3-68 × 107 d m 3 mole -1 s -1. These data indicate that ks, are s o m e h o w concentration d e p e n d e n t , but the value 107 d m 3 mole -1 s -1 seems to be reasonable. P o l y ( 1 - ( 4 - b i p h e n y l ) - 2 - p r o p e n - l - o n e ) p r e p a r e d by radical polymerisation has M , = 1.14 × 105. 2 Using the M a r k - H o u w i n k constants for p o l y ( 1 - ( 1 - p h e n y l ) - 2 - p r o p e n - l - o n e ) ~ the limiting viscosity n u m b e r of this p o l y m e r is 5 0 c m 3 g -1. T h e local concentration of 4-biphenylcarbonyl c h r o m o p h o r e s in the region of the m a c r o m o l e c u lar coil can be calculated using the relationship: 9 1-2 × 103w CL
['o]M
w h e r e [a~] is the limiting viscosity n u m b e r ; w is the mass fraction of the c h r o m o p h o r e in the p o l y m e r (in the case of h o m o p o l y m e r w - 1) and M the molar mass of the structural unit of the c h r o m o p h o r e . The local concentration for the h o m o p o l y m e r (0.116 mole dm -3) is substantially higher than the total concentration (10 - 3 - 10 -4 mole din-3). Substituting all the known p a r a m e t e r s in the S t e r n - V o l m e r equation 4)-2°= 1 + k ~ c L
¢
126
Pavol Hrdlovic, Ghislain Guyot, Jacques Lemaire, Ivan Lukac
I
i
l
l
!
i i i
AA
!
"l
i
0,1
! I
i
i l
#
l
!
I
i
I ®
\
~ /~ t~._.O,, ~4 ~¢./. . . . . . . ~ N . \ .r
"~.-.... - ~..~..
,,..~_
x~. 2"_~.~.,.~.._ ....
.
0
Fig. 2. The transient spectrum in benzene at 10-3 mole dm-a of (1) 1-(4-biphenyl)3-chloro-l-propanone; (2) poly(1-(4-biphenyl)-2-propen-l-one)--anionic; (3) poly(1-(4-biphenyl)-2-propen-l-one)--radieal; (4) poly(1-(4-biphenyl)-2-propen-l-oneco-methyl methacrylate).
for k~ = 107 d m s mole -1 s-l; "r = 0 . 2 8 6 ms; CL = 0"116 mole d m - 3 then the ratio 4>/4,0 is 0.003. C o m p a r i s o n of the transient spectrum at its m a x i m u m for the c o p o l y m e r and radical h o m o p o l y m e r shows that the ratio is 0.1 (Fig. 2). This rough calculation at least qualitatively indicates that self-quenching does not o p e r a t e to the fullest extent in the h o m o p o l y m e r . A p p r o x i m a t e l y one-thirtieth of the total content of the c h r o m o p h o r e s present takes part in the self-quenching. T h e self-quenching is even less p r o n o u n c e d for the anionic h o m o p o l y m e r . 6 T h e molecular mass of the anionic poly(1-(4b i p h e n y l ) - 2 - p r o p e n - l - o n e ) was lower than that of the radical one. T h e differences in the emission spectra of the radical and anionic h o m o p o l y m e r s were explained b y the different n u m b e r of p e r t u r b a tions in the macromolecular chain as a result of its different moleculax mass. 2 Perhaps these two effects are interdependent.
4-Biphenylcarbonyl and 2-naphthylcarbonyl chromophores
127
The comparison of 4-biphenylcarbonyl and 2-naphthylcarbonyl chromophores shows that self-quenching is more pronounced for the last one. Owing to self-quenching no transient is observed for poly(1(2-napthyl)-2-propen-l-one) or for the copolymer poly(1-(2naphthyl)-2-propen-l-one-co-methyl methacrylate) with a 10 mass/mass % of carbonyl monomer. The transient spectrum of the respective model compound was extremely weak in carbon tetrachloride at 1 0 - 3 mole dm -3. A more intense spectrum was observed at a lower concentration 1 0 -4 mole dm -3, in the region 400-600 nm (Fig. 3). Around 360 nm an additional long-lived component is observed which was not present in benzene. Similar anomalies were observed in the flash photolysis study of 2-acetylnaphthalene. 1° Although the self-quenching of the 2-naphthyl carbonyl chromophore cannot be quantitatively evaluated from our data, it is quite clear that the 4-biphenylcarbonyl chromophore is less sensitive to selfquenching. Therefore, this chromophore can be utilised as a triplet
AA 0,1
r~
;'
1
I
400
.500
600
.~ ,nm
700
Fig. 3. The transient spectrum of poly(1-(4-biphenyl)-2-propen-l-one-co-methyl methacrylate) at 1 0 - 4 m o l e d m -3 in (1) benzene; (2) a mixture of benzeneisopropanol (2: 3).
128
Pavol Hrdlovic, Ghislain Guyot, Jacques Lemaire, Ivan Lukac
probe. Recently, 1,2-dicarbonyls 11'~2 have been used as triplet probes for studying the kinetics and dynamics of the macromolecular chain. For similar purposes 4-biphenylcarbonyl can be employed but the formation and decay of the triplet state must be detected by absorption spectroscopy. This type of detection, however, is less sensitive than emission spectroscopy. The transient spectrum and the life-time unequivocally prove that the highest concentration of the triplet state for 4-biphenylcarbonyl chromophore is reached in a copolymer of poly(1-(4-biphenyl)-2propen-l-one-co-methyl methacrylate) with a low chromophore content 4 mass/mass % in a diluted solution where intermacromolecular interactions are minimalised. In a thermodynamically good solvent the macromolecular coil is expanded and consequently the number of intramolecular interactions is low, too. Self-quenching is negligible under these conditions. The slightly lower value of the life-time at 10 -4 mole d m -3 is probably caused by the presence of oxygen to which the chromophore is more sensitive at lower than at higher total concentration. The addition of a more polar solvent (precipitant) results in a higher number of intramolecular contacts and more efficient self-quenching leading to a lower triplet life-time (Table 1). The conclusions presented in this paper are qualitative only. For quantitative conclusions more data are needed. Nevertheless, we could show the possibilities of employing a 4-biphenylcarbonyl triplet probe for the study of a macromolecular coil. Self-quenching as an important deactivation process of the triplet state of poly(4-vinylbenzophenone) was shown to occur at lower flash intensities. 13 The decay in this case was of the second order for polymer in comparison with the model compound. In our polymer system, a first-order decay was observed exclusively. The main differences between these systems are: (1)
(2)
The life-time of the benzophenone triplet in the homopolymer and model compound is in the range 0.7-5 ms which is substantially longer than that for 4-biphenylcarbonyl and the 2-naphthylcarbonyl chromophore. The self-quenching rate constant for benzophenone chromophore is 105dm3mole-ls -1 while for 4-biphenylcarbonyl it was estimated to be 107 dm 3 mole -1 s-x and for 2naphthylcarbonyl even higher.
4-Biphenylcarbonyl and 2-naphthylcarbonyl chromophores
129
T h e reason for no T - T annihilation and first-over decay in our systems might be explained by these differences; in any case, a m o r e detailed analysis is n e e d e d and will be the subject of further studies.
ACKNOWLEDGEMENTS O n e of the authors (P.H.) thanks Club E D F 'Photochimie' for financial support.
REFERENCES 1. Hrdlovic, P. and Lukac, I., In: Developments in polymer degradation, Vol. 4, Grassie, N., in press. 2. Lukac, I. and Hrdlovic, P., European Polym. J., 14 (1978) 339. 3. Selwyn, J. C. and Scaiano, J. C., Polymer, 21 (1980) 1365. 4. Faure, J., Fouassier, J. P., Lougnot, D. J. and Salvin, R., Nouveau J. Chim., 1 (1977) 15. 5. Beck, G., Dobrowolski, G., Kiwi, J. and Schnabel, W., Macromolecules, 8 (1975) 9. 6. Hrdlovic, P., Trekoval, J. and Lukac, I., European Polym. J., 15 (1979) 229. 7. Jeandrau, J. P., Gramain, J. C. and Lemaire, J., J. Chem. Res. Synop., (1979) 186. 8. Merkel, P. B. and Kearns, D. R., J. Chem. Phys., 58 (1973) 398. 9. Sanchez, G., Knoesel, R. and Weill, G., European Polym. J., 14 (1978) 485. 10. Schuster, D. J. and Goldstein, M. D., Molecular Photochem., 7 (1978) 209. 11. Horie, K. and Mita, I., Polym. J., 9 (1977) 201. 12. Horie, K. and Mita, I., Macromolecules, 11 (1978) 1175. 13. Schnabel, W., Makromol. Chemie, 180 (1979) 1487.
Triplet State of 4-Biphenylcarbonyl and 2-Naphthylcarbonyl Chromophores in Homopolymers, Copolymers and Model Compounds Pavol Hrdlovict, Ghislain Guyot,$ Jacques Lemaire$ and Ivan Lukact t Polymer Institute of the Slovak Academy of Sciences, CS-842 36 Bratislava, Dubravska cesta, Czechoslovakia ~:Laboratoire de Photochimie Mol6culaire et Macromol6culaire (6quipe associ6e au CNRS no. 929), Universit6 de Clermont II, B.P. 45, 63170 Aubiere, France (Received: 22 March, 1982)
ABSTRACT The triplet state of poly(1-(4-biphenyl)-2-propen-l-onc), poly(1-(2naphthyl)-2-propen-l-one), copolymers of the above carbonyl monomers with methyl methacrylate and respective model compounds was sought in solution at room temperature by emission spectroscopy and microsecond ]lash photolysis. Although 4-biphenylcarbonyl and 2-naphthylcarbonyl yield an intense phosphorescence at 77 K, no emission was observed in solution at room temperature. The intense transient spectrum observed in benzene solution for 1-( 4-biphenyl)-3chloropropan-l-one and 1-(2-naphthyl)-3-chloropropan-l-one was ascribed to the triplet state. Self-quenching was found to decrease the intensity of the transient spectrum and the life-time when building up both chromophores in a macromolecule. More e]~cient self-quenching was observed for 2-naphthylcarbonyl than for the 4-biphenylcarbonyl chromophore which was suggested as a suitable triplet probe. The limiting triplet life-time of 4-biphenylcarbonyl built up in a copolymer with methyl methacrylate at low concentration in a thermodynamically good solvent is about 0-3 ms. INTRODUCTION T h e r e l e v a n t d a t a of t he p h o t o p h y s i c a l and p h o t o c h e m i c a l processes of p o l y ( 1 - ( 4 - s u b s t i t u t e d p h e n y l ) - 2 - p r o p e n - l - o n e ) s , c o p o l y m e r s of t he 119
Polymer Photochemistry 0144-2880/83/0003-0119/$3.00 © Applied Science Publishers Ltd, England, 1983. Printed in Northern Ireland
120
Pavol Hrdlovic, Ghislain Guyot, Jacques Lemaire, Ivan Lukac
respective carbonyl m o n o m e r s with styrene and methyl methacrylate and of the model compounds have been recently reviewed. 1 It follows from this review that the electron-donating substituents in position 4 shift the low temperature emission bathochromically with disappearance of vibrational structure, prolong the life-time and strongly decrease the photochemical reactivity of the Norrish type II reaction in solution at room temperature. L2 This conclusion is supported by the laser flash photolysis measurements which showed that the lifetime for poly(1-(4-methoxyphenyl)-2-propanol-one), i.e. 800 n s , 3 is substantially longer than that for the unsubstituted homopolymer, i.e. 7-70 n s . 4"5 In this paper, we describe our attempts to characterise the triplet state of homopolymers, copolymers and model compounds of this type with 4-diphenylcarbonyl and 2-naphthylcarbonyl chromophores at room temperature in solution using emission spectroscopy and microsecond flash photolysis.
EXPERIMENTAL Model compounds, homopolymers and copolymers of carbonyl m o n o m e r s with methyl methacrylate are described in a previous paper, z Copolymer poly(1-(4-biphenyl) 2-propen-l-one-comethylmethacrylate) with 4 mass/mass % of carbonyl m o n o m e r was prepared in the same way as in Reference 2. H o m o p o l y m e r poly(1(4-biphenyl)-2-propen-l-one) was prepared by anionic polymerisation as described in Reference 6. B e n z o p h e n o n e (analytical reagent, Suchard), acetophenone, propiophenone, 4-methoxyacetophenone (purest grade, Fluka AG) were used without further purification. Emission spectra were taken on a Perkin Elmer M F P - 3 A instrum e n t from degassed carbon tetrachloride solutions (spectral grade, Merck). The solutions were degassed by seven-fold freeze-thaw cycles under high vacuum. The transient spectra and the decays were measured on a flash photolysis apparatus FPX-1 (Nortech Laboratories Ltd, England) in microsecond range. The photomultiplier output was fed to a Transient Recorder D L 905 (Data Laboratories Ltd) and recorded either on an oscilloscope Telequipment D 61a or on a recorder.
4-Biphenylcarbonyl and 2-naphthylcarbonyl chromophores
121
RESULTS A N D DISCUSSION Emission measurements In the previous p a p e r s 1'2 it was shown that poly(1-(4-substituted phenyl)-2-propen-l-one)s and their respective model compounds yield intense phosphorescence at 77 K in film or solid solution. Usually the n-~r* triplet is short-lived but the phosphorescence coming from the 1r-lr* triplet lives more than 100 ms. 1-3 In the experimental s e t - u p , 7 it was easy to observe the phosphorescence arising from the n - w * triplet level for benzophenone, propiophenone and acetophenone in solution at room temperature as in Reference 8. The introduction of 4-methoxy group in the acetophenone type molecule leads to a decrease in the quantum emission yield. At the highest sensitivity, no emission was observed for 4-methoxyacetophenone, 1-(4-methoxyphenyl)-3-chloro-1propanone, 1-(4-biphenyl)-3-chloro- 1-propanone and 1-(2naphthyl)-3-chloro-l-propanone. This shows that the radiationless transition of the triplet state depends on the temperature in a different way for n-~r* and ~--1r* levels. The emission of the lowest triplet state can be observed even at room temperature, when described as an n-Tr* type configuration. The emission of the lowest 7r-Tr* triplet state vanishes when the temperature increases from 77 K to 298 K. Emission measurements yield no information concerning the triplet states of chromophores under investigation. Flash photolysis measurements Flash photolysis in the microsecond region revealed that model compounds of 4-biphenylcarbonyl and 2-napthylcarbonyl chromophores yield a transient with a maximum around 400-425 nm. The relevant spectra and kinetic data of the transient spectra are summarised in Table 1. The intensity of the flash is proportional to the energy of the system U = ½CV2 where C is the charging capacity of the system and V is the voltage. The linear correlation between the flash intensity and concentration of the excited species indicates the involvement of a monophotonic process followed by a first-order decay, as the maximal intensity of the signal is considered as the initial intensity.
122
P a v o l Hrdlovic, G h i s l a i n G u y o t , J a c q u e s L e m a i r e , I v a n L u k a c
'i'
7
X
X ¢~
X
II
II
II
0 0
~g e~
<
i
?
9
0
O O 0 0 0 ~
9
I
z,
,
?
~
¢.J
-.z, ~.z, ~ .._, ~ v
44444N
I
nJ
8.~8~8
4-Biphenylcarbonyl and 2-naphthylcarbonyl chromophores
o
.=.
i
d~
~8
d~
?"
~'
~B 0
o
o
~ ??7?
z,~z
o
o
o
~Z~ a ~',.~
&~&
&
,~e,~
.o
&
.£ -
.-.
'rg
•o
°,o'~
,1= o
o
o
123
124
Pavol Hrdiovic, Ghislain Guyot, Jacques Lemaire, Ivan Lukac
0,3
AA
0,1
2
'
V2
'
26o
Fig. 1. The dependence of the absorbance at 425 nm of poly(1-(4-biphenyl)-2propen-l-one-co-methyl methacrylate) at 1 0 - 4 m o l e d m -3 on the intensity (1) in benzene; (2) in a mixture of benzene-isopropanol (2 : 3).
Figure 1 shows an excellent correlation of this type (line 1). If self-quenching takes place, there is a strong deviation from linearity at higher flash intensities (line 2). The decay is, in this case, still of pseudo-first-order because the n u m b e r of excited states seems to be lower than the n u m b e r of unexcited molecules. Since no p h o s p h o r e s c e n c e was observed from the above comp o u n d s at r o o m t e m p e r a t u r e in solution, it is necessary to identify the observed transient as a triplet or a radical. T h e following observations indicate that p r o b a b l y the lowest long-lived triplet is involved. 1.
2.
The life-time of the transient and the maximal intensity of the signal decreases as the concentration of the substrate increases in the range 1 0 - 4 - 1 0 -3 mole dm -3. This does not exclude a radical as a possible intermediate b u t the decay is always of the pseudo-first-order, which more satisfies a triplet self-quenching than a bimolecular decay for a radical. A n excess of a solvent with an easily abstractable hydrogen, i.e. isopropanol, influences only slightly the transient spectrum
4-Biphenylcarbonyl and 2-naphthylcarbonyl chromophores
125
of the model c o m p o u n d . Consequently, the formation of a radical does not occur. 3. The transient signal is q u e n c h e d by the addition of 4acetyloxy-2,2,6,6,-tetramethylpiperidine-N-oxyl which, however, is not consumed, as indicated by E S R measurements. Consequently, this radical acts as a triplet quencher. T h e quenching rate constants were calculated from the measurements at a single concentration. A t higher quencher concentration, the quenching rate constants were lower. T h e important r o u t e for the deactivation of the triplet state of the c h r o m o p h o r e s under study is self-quenching. Building up of the 4-biphenylcarbonyl c h r o m o p h o r e in a macromolecule leads either to an increased self-quenching for h o m o p o l y m e r or to a full exclusion of the self-quenching for c o p o l y m e r with methyl methacrylate with a low content of the a b o v e c h r o m o p h o r e (Fig. 2). Assuming the triplet life-time for c o p o l y m e r p o l y ( 1 - ( 4 - b i p h e n y l ) - 2 - p r o p e n - l - o n e - c o methyl methacrylate) to be 0-286 ms, then the rate constant of self-quenching k,q can be calculated, for the concentration 10 -3 and 10 -4 mole d m -3, respectively 5 - 8 4 × 106 d m 3 m o l e -1 s -1 and 3-68 × 107 d m 3 mole -1 s -1. These data indicate that ks, are s o m e h o w concentration d e p e n d e n t , but the value 107 d m 3 mole -1 s -1 seems to be reasonable. P o l y ( 1 - ( 4 - b i p h e n y l ) - 2 - p r o p e n - l - o n e ) p r e p a r e d by radical polymerisation has M , = 1.14 × 105. 2 Using the M a r k - H o u w i n k constants for p o l y ( 1 - ( 1 - p h e n y l ) - 2 - p r o p e n - l - o n e ) ~ the limiting viscosity n u m b e r of this p o l y m e r is 5 0 c m 3 g -1. T h e local concentration of 4-biphenylcarbonyl c h r o m o p h o r e s in the region of the m a c r o m o l e c u lar coil can be calculated using the relationship: 9 1-2 × 103w CL
['o]M
w h e r e [a~] is the limiting viscosity n u m b e r ; w is the mass fraction of the c h r o m o p h o r e in the p o l y m e r (in the case of h o m o p o l y m e r w - 1) and M the molar mass of the structural unit of the c h r o m o p h o r e . The local concentration for the h o m o p o l y m e r (0.116 mole dm -3) is substantially higher than the total concentration (10 - 3 - 10 -4 mole din-3). Substituting all the known p a r a m e t e r s in the S t e r n - V o l m e r equation 4)-2°= 1 + k ~ c L
¢
126
Pavol Hrdlovic, Ghislain Guyot, Jacques Lemaire, Ivan Lukac
I
i
l
l
!
i i i
AA
!
"l
i
0,1
! I
i
i l
#
l
!
I
i
I ®
\
~ /~ t~._.O,, ~4 ~¢./. . . . . . . ~ N . \ .r
"~.-.... - ~..~..
,,..~_
x~. 2"_~.~.,.~.._ ....
.
0
Fig. 2. The transient spectrum in benzene at 10-3 mole dm-a of (1) 1-(4-biphenyl)3-chloro-l-propanone; (2) poly(1-(4-biphenyl)-2-propen-l-one)--anionic; (3) poly(1-(4-biphenyl)-2-propen-l-one)--radieal; (4) poly(1-(4-biphenyl)-2-propen-l-oneco-methyl methacrylate).
for k~ = 107 d m s mole -1 s-l; "r = 0 . 2 8 6 ms; CL = 0"116 mole d m - 3 then the ratio 4>/4,0 is 0.003. C o m p a r i s o n of the transient spectrum at its m a x i m u m for the c o p o l y m e r and radical h o m o p o l y m e r shows that the ratio is 0.1 (Fig. 2). This rough calculation at least qualitatively indicates that self-quenching does not o p e r a t e to the fullest extent in the h o m o p o l y m e r . A p p r o x i m a t e l y one-thirtieth of the total content of the c h r o m o p h o r e s present takes part in the self-quenching. T h e self-quenching is even less p r o n o u n c e d for the anionic h o m o p o l y m e r . 6 T h e molecular mass of the anionic poly(1-(4b i p h e n y l ) - 2 - p r o p e n - l - o n e ) was lower than that of the radical one. T h e differences in the emission spectra of the radical and anionic h o m o p o l y m e r s were explained b y the different n u m b e r of p e r t u r b a tions in the macromolecular chain as a result of its different moleculax mass. 2 Perhaps these two effects are interdependent.
4-Biphenylcarbonyl and 2-naphthylcarbonyl chromophores
127
The comparison of 4-biphenylcarbonyl and 2-naphthylcarbonyl chromophores shows that self-quenching is more pronounced for the last one. Owing to self-quenching no transient is observed for poly(1(2-napthyl)-2-propen-l-one) or for the copolymer poly(1-(2naphthyl)-2-propen-l-one-co-methyl methacrylate) with a 10 mass/mass % of carbonyl monomer. The transient spectrum of the respective model compound was extremely weak in carbon tetrachloride at 1 0 - 3 mole dm -3. A more intense spectrum was observed at a lower concentration 1 0 -4 mole dm -3, in the region 400-600 nm (Fig. 3). Around 360 nm an additional long-lived component is observed which was not present in benzene. Similar anomalies were observed in the flash photolysis study of 2-acetylnaphthalene. 1° Although the self-quenching of the 2-naphthyl carbonyl chromophore cannot be quantitatively evaluated from our data, it is quite clear that the 4-biphenylcarbonyl chromophore is less sensitive to selfquenching. Therefore, this chromophore can be utilised as a triplet
AA 0,1
r~
;'
1
I
400
.500
600
.~ ,nm
700
Fig. 3. The transient spectrum of poly(1-(4-biphenyl)-2-propen-l-one-co-methyl methacrylate) at 1 0 - 4 m o l e d m -3 in (1) benzene; (2) a mixture of benzeneisopropanol (2: 3).
128
Pavol Hrdlovic, Ghislain Guyot, Jacques Lemaire, Ivan Lukac
probe. Recently, 1,2-dicarbonyls 11'~2 have been used as triplet probes for studying the kinetics and dynamics of the macromolecular chain. For similar purposes 4-biphenylcarbonyl can be employed but the formation and decay of the triplet state must be detected by absorption spectroscopy. This type of detection, however, is less sensitive than emission spectroscopy. The transient spectrum and the life-time unequivocally prove that the highest concentration of the triplet state for 4-biphenylcarbonyl chromophore is reached in a copolymer of poly(1-(4-biphenyl)-2propen-l-one-co-methyl methacrylate) with a low chromophore content 4 mass/mass % in a diluted solution where intermacromolecular interactions are minimalised. In a thermodynamically good solvent the macromolecular coil is expanded and consequently the number of intramolecular interactions is low, too. Self-quenching is negligible under these conditions. The slightly lower value of the life-time at 10 -4 mole d m -3 is probably caused by the presence of oxygen to which the chromophore is more sensitive at lower than at higher total concentration. The addition of a more polar solvent (precipitant) results in a higher number of intramolecular contacts and more efficient self-quenching leading to a lower triplet life-time (Table 1). The conclusions presented in this paper are qualitative only. For quantitative conclusions more data are needed. Nevertheless, we could show the possibilities of employing a 4-biphenylcarbonyl triplet probe for the study of a macromolecular coil. Self-quenching as an important deactivation process of the triplet state of poly(4-vinylbenzophenone) was shown to occur at lower flash intensities. 13 The decay in this case was of the second order for polymer in comparison with the model compound. In our polymer system, a first-order decay was observed exclusively. The main differences between these systems are: (1)
(2)
The life-time of the benzophenone triplet in the homopolymer and model compound is in the range 0.7-5 ms which is substantially longer than that for 4-biphenylcarbonyl and the 2-naphthylcarbonyl chromophore. The self-quenching rate constant for benzophenone chromophore is 105dm3mole-ls -1 while for 4-biphenylcarbonyl it was estimated to be 107 dm 3 mole -1 s-x and for 2naphthylcarbonyl even higher.
4-Biphenylcarbonyl and 2-naphthylcarbonyl chromophores
129
T h e reason for no T - T annihilation and first-over decay in our systems might be explained by these differences; in any case, a m o r e detailed analysis is n e e d e d and will be the subject of further studies.
ACKNOWLEDGEMENTS O n e of the authors (P.H.) thanks Club E D F 'Photochimie' for financial support.
REFERENCES 1. Hrdlovic, P. and Lukac, I., In: Developments in polymer degradation, Vol. 4, Grassie, N., in press. 2. Lukac, I. and Hrdlovic, P., European Polym. J., 14 (1978) 339. 3. Selwyn, J. C. and Scaiano, J. C., Polymer, 21 (1980) 1365. 4. Faure, J., Fouassier, J. P., Lougnot, D. J. and Salvin, R., Nouveau J. Chim., 1 (1977) 15. 5. Beck, G., Dobrowolski, G., Kiwi, J. and Schnabel, W., Macromolecules, 8 (1975) 9. 6. Hrdlovic, P., Trekoval, J. and Lukac, I., European Polym. J., 15 (1979) 229. 7. Jeandrau, J. P., Gramain, J. C. and Lemaire, J., J. Chem. Res. Synop., (1979) 186. 8. Merkel, P. B. and Kearns, D. R., J. Chem. Phys., 58 (1973) 398. 9. Sanchez, G., Knoesel, R. and Weill, G., European Polym. J., 14 (1978) 485. 10. Schuster, D. J. and Goldstein, M. D., Molecular Photochem., 7 (1978) 209. 11. Horie, K. and Mita, I., Polym. J., 9 (1977) 201. 12. Horie, K. and Mita, I., Macromolecules, 11 (1978) 1175. 13. Schnabel, W., Makromol. Chemie, 180 (1979) 1487.