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Excimer intermediate in the unsymmetrical photodimerization of the anthracene ring system
CHEMICAL PHYSICS
Volume 78, number 3
15 March 1981
LETTERS
EXCIMER INTERMEDIATE IN THE ~S~~E~~ICAL OF THE ANTHRACENE RING SYSTEM
P~OTO~~EK~ZATrO~
James FERGUSON Research School of Cltctnistry. Australian National Umiwstry,
Carzberru, A C T 2600, Australia
and Alain CASTELLAN.
Jean-Pierre
I3ESVERGNE
Laboratoire de Cjlimie orgamqtte ct II: R A 80 33405 Takwce lkme Recewed
29 November
Mtasurcmcnts mcthyld~sdo~ane
167
and Henri BOUAS-LAURENT Univcrsit& de Bordeasx 1.
1980
of fiuorescence spectra and fiuorescence quantum yields sssocznted wiGx 1,3-bts(9-antIuyl)-l,i,3,3-tcuaand rts photozsomer indwzstc that photochemistry occurs from an unsymmetrical exctmer
l_ Introduction Unsymmetrrcal pllot~dimerizatiort of the anthracene rmg system has been demonstrated recently by BouasLaurent and co-workers [ 1,2] _ In view of the recent discuswons concermng the mechamsm of the symmetrical photodimerization of the anthracene ring system in a,u-bis-(9-antflryl)-rr-alkanes, a quantitative study of the unsymmetrical photoreaction is desirable [3,4] in order to establish whether this reaction proceeds throu& an excimer state [5-8]_ Both compounds reported by Bouas-Laurent and co-workers [I,21 have been studied in the present work, but the essential features of the reversible unsymmetric& photoreaction are covered adequately by consideration of only one compound. The present communication therefore reports a study of the photoisomerization of 1,3-bis-(9anthryl)-I, 1.3,3-tetramethyldisiloxane (I,) and the photodissociation of its photoisomer (i,), characterized by the unsymmetrical 9, I ‘; 10,4’ bonds (see scheme I). A-
Me2Si
- O-Si
Particular attention has been paid to the measurement of the quantum yields of fluorescence of i, and I,, over a range of temperature. down to 100 K in a glass forming solvent composed of methylcyclohexane and isopentane (MCI-I : IP, 1 : 3). in addition, the fluorescence spectra have been measured over the same range. The various data show that fluorescence and photocycioaddition are in competition in an extimer state which has an unsymmet~c~ conformation of the two anthracene chromophores.
2. Experimental The preparation of I, has been described [‘_I _ f, was obtained in situ, by irradiation (wavelengths greater than 340 nm) of I, in solution in evacuable
Me+
A = 9_ anthryl
fcl 446
lb
Scheme I 0 009--2614/8
1/0000-0000/S
02.50
0 North-Holland Publishing Company
CHEMICAL
Volume 78, number 3
1.5 March 1981
PHYSICS LETTERS
square cross section silica absorption celIs, degassed by several freeze-thaw cycles_ Fluorescence spectra and quantum yields were determined by methods already outlined [3] _
3. Results and discussion 3.1. Fzuort?scer2ceof
la
The fluorescence of I, shows, at room temperature, features characteristic of the 9-anthryl chromophore (localized emission, M), as well as a broad band characteristic of excimers (E) [2] _ Lowering of the temperature leads to an increase of both M and E components mitially, followed by a decrease of the E component and a continued increase of the M component. Fig. 1 shows a selection of fluorescence spectra_ There are two components to the E emission. E, with a maxiFig. 2. Temperature variation of the total fluorescence yield (@M+E), the fluorescence yield of the locahzed chromophore (on& the fluorescence yreld of the exc~mer (+) and the fluorescence quenching yield (0~) of I, 111MCH t IP.
mum near 2 1000 cm-’ and E2 at = 18 900 cm-‘. On warming from 100 K, El appears first, then El at a higher temperature_ These two excimers correspond to the two observed by Castellan et al. [2] from photodissociated Ib in MCH at 77 K. The fluorescence yield of I, varies markedly with temperature. Fig. 2 shows the variation of total yield (#hX+E), the localized fluorescence yield ($J~), the extimer emission yield (QQE)and the quenching yield QQ) obtained by difference (1 - dlM+&_
”
c i
3.2. Photochemistyy
ofi;, and I,
The quantum yield of photocycloaddition
MCH - J.Pwas measured at room temperature
of I, in
and
found to be 0.17. Excitation was at 389 nm. The quantum yield of photodissociation of 1, in MCH : IP at room temperature and irradiation at 290 nm was 0.81. 24
Fig. 1. Fluorescence spectra of Ia recorded at various temperatures in MCH : IP.
3.3. Fhorescence
of 1,
Irradiation of $, in MCH : IP gives rise to fluorescence which changes with temperature. At -100 K 447
CHEMICAL PHYSICS LETTERS
Volume 78. number 3
15 March 1981
J-
5-
3-
5-
21,1*3
22
20
18
16
CM-’ Frg. 3. Elcited-state product fluorescence spectra obtnmed by irradratron of tb in %lCH IP at 290 mu at various temperatures the emissron IS characteristic of the naphthalene chromophore and lies in the ultraviolet _At = 120 K there appears a broad exctmer band in the visible
500 cm-’ (fig. 3). Further increase of temperature leads to an mcrease of the excimer fluorescence intensity and a drop of the ultraviolet band intensity_ The excimer intensrty reaches its maximum at -225 K and then drops. The fluorescence quantum yields were determined and the naphthalene chromophore yield (@b), the extimer yield (0;) and the photoreaction yield (@k = 0.2 - &I - @ are given in fig 4. reSion at -18
3 4. AHQZYS~S Whereas the (ground-state) conformation responsible for the M emissron of I, at 100 K has unit quantum
efficiency, the (excited-state) conformation which provides the El emrssion has a non-zero internal quenching yieId. This is shown by the simultaneous rise of Q. and QB at 2130 K. With the appearance of E?, GE levels off at -160 K and the rate of increase of QQ drops. The data in figs. 1 and 2 suggest the presence of three different excited-state conformations, two of which have some overlap of the two anthracene chromophores and are popuiated via relaxation from the (ground-state) conformation which gives rise to the M 448
- ._
,_/yy----. 200
3c
T l-1: 4. Quantum yields obtained by rrradiation of Ib in MCH IP at 290 m oil is the naphthalene chromopbore fluorescence yreld, ok IS the elcimer excited-state product fluorescence yield nnd 0’R is the photoreaction yieId (0.2.-& - &)
fluorescence Relaxation to the El conformation cannot involve large movements of the chromophores because the onset of its fluorescence is at 130 K. Previous work [9] has shown that in MCH : IP, rotational reiaxatron leading to depolarization of fluorescence, takes pIace at =I20 K. so that only small rotationa motions are hkely in the relaxation from M to El conformations Transition to the second excimer conformation I&) takes place at =I45 K and the total excimer intensity remains nearly constant until -2 10 K and then it falls. Turning to the data for Ib (fig. 4), we see that quenching of the ultraviolet fluorescence begins at = 115 K and at the same time there is a growth of the excimer fluorescence (fig. 3). Between 100 and 210 K the E emission grows at the expense of the M emission and there is no quenching of the total fluorescence yield. Above 210 K however, quenching is observed and, by 230 K, the emission of the excimer has reached its maximum intensity and then it falls, similar to that observed for I,. It is natural to conclude that the quenching process is the same in each case, i.e. photocycloaddition. AdySiS of the data is simpler for Ib because of the absence of emission from the 9-anthryl chromophore. The following scheme includes only
Volume 78, number 3
CHEMICAL PHYSICS LETTERS
those reactions which are significant the interpretation of the data. 1, +hv+
and necessary
g,
(1)
kMF Ib* __, I, +hv’,
km
12-
(2)
I,*(E),
kEF
I,*(E)---+
for
(3)
Ia + kv”,
(4)
(-9 The quantum efficiency of photocycloaddiiion from the excimer (OR) is the ratio of the rate of photocycloaddition to the rate of disappearance of the extimer. It follows from (l)-(5), using steady-state conditions,
that
\ve assume that kR is temperature k,
dependent,
= A exp(-AE?/RT),
(7)
so that
6,’ -
1 = (k&A)
exp(Ai?/RT).
The data are plotted
in the Arrhenius
(8) form (8) in
15 March 1981
fig. 5. The broken line is a least-squares fit and provides kE/A = 2.5 X 10s7 and AE* = 79 kal mol-I_ Next we treat the data in fig. 2 usmg the approximations of Chandross and Dempster [lo], i.e. we have plotted lr1(1~/1~ - 1) against l/T_ Here Irnm is the excimer intensity at 210 K and IT is the intensity of the excimer emission at temperature T (210 < T< 298 K). The plot is shown in fig. 5 along with the least-squares straight line. Af? = 8.6 kcal mol-’ in good agreement with the value obtained previously (within experimental errors)_ The direct determination of the quantum yitld of photocycloaddition of 1a gave a value of 0.17 and we see from fig. 2 that the difference between the roomtemperature @a and the intercept obtained by a smooth extrapolation of the low-temperature QR curve (broken curve) IS ~0.17. The mcreased quenching above -230 K is accounted for by photocycloaddition, which competes with fluorescence from the E, conformation. It remains to establish the likely arrangement of the two chromophores in the E, conformation_ Although this cannot be done by a direct method, it is known from the work of Hayashi et al. [ 111 that [2.2] (1,4)(9,lO)anthracenophane (II) is both fluorescent and photoreactive. It therefore has the two chromophores fuced in an unsymmetrical arrangement and a weak room-temperature fluorescence was found [ 111 which became stronger on cooling to 77 K. The maximum of the 77 K fluorescence spectrum was 570 nm (=17 500 cm-‘), which is close to the 140 K fluorescence intensity maximum of the E, extimer (fig. 3). The difference can be ascribed to the effect of the two methylene bridges in II. It seems highly likely therefore, that the conformation E2, not E, as suggested previously [2], of I,, the photoreactive conformation, has an unsymmetrical arrangement of the two anthracene chromophores, in which the planes should be essentially parallel in analogy with previous results [ 1 l] _Photoreaction proceeds from this excimer state.
Acknowledgement Fiz. 5. Arrhenius plots of the quantum efficiency of photocycloaddition from the evcimer obtained by irradiation of Ib (right-hand scale) and the quenching of the excimer fluorescence of Ia (left-hand scale).
We thank F-C. de Schryver for helpful comments and for a careful reading of the manuscript.
449
Volume
78, number
3
CHEMICAL
PHYSICS
References f l] G. Felix, R. Lapouyade, H Bows-Laurent and B, CJin, Tetrahedron Letters (1976) 2277. [ 21 A. CasteBan, J.-P_ Desvergne and H. Bows-Laurent, Nouv. J. Chim 3 (1979) 231. [3] J. Ferguson, Chem Phys. Letters 76 (1980) 398. [4] G. Jones II, ‘W-R. Bergrnark and A.M. Halpern, Chem. Phys Letters 76 (1980) 403. [5] J. Ferguson and A-W.-H. Mau, Mol. Phys. 27 (i974) 377.
450
LETTERS [6] ]7] [8J [9]
[lo] fl l]
15 March 1981
N. Boens, M de Brackeleire, J. Huybrechts and F.C. de Schryver, 2. Physik. Chem. NF 101 (1976) 417_ G. K~UPP. Ann. Chem. (1973) 844. W-R. Begmark, G. Jones If, T E. Reinhardt and A M. Halpern, J. Am. Chem. Sot. 100 (1978) 6665. J. Ferguson, A.W.-H. Mau and P.O. Whimp, J. Am. Chem Sot lOI(1979) 2370 E A. Chandross and C J. Dempster, J. Am. Chem. Sot. 92 (1970) 3.568. T. Hayashr, N. Mataga, Y. Sakata, S. Misumi, M_ Monta and J. Tanaka, J. Am. Chem. Sot. 98 (1976) 5910.
Volume 78, number 3
15 March 1981
LETTERS
EXCIMER INTERMEDIATE IN THE ~S~~E~~ICAL OF THE ANTHRACENE RING SYSTEM
P~OTO~~EK~ZATrO~
James FERGUSON Research School of Cltctnistry. Australian National Umiwstry,
Carzberru, A C T 2600, Australia
and Alain CASTELLAN.
Jean-Pierre
I3ESVERGNE
Laboratoire de Cjlimie orgamqtte ct II: R A 80 33405 Takwce lkme Recewed
29 November
Mtasurcmcnts mcthyld~sdo~ane
167
and Henri BOUAS-LAURENT Univcrsit& de Bordeasx 1.
1980
of fiuorescence spectra and fiuorescence quantum yields sssocznted wiGx 1,3-bts(9-antIuyl)-l,i,3,3-tcuaand rts photozsomer indwzstc that photochemistry occurs from an unsymmetrical exctmer
l_ Introduction Unsymmetrrcal pllot~dimerizatiort of the anthracene rmg system has been demonstrated recently by BouasLaurent and co-workers [ 1,2] _ In view of the recent discuswons concermng the mechamsm of the symmetrical photodimerization of the anthracene ring system in a,u-bis-(9-antflryl)-rr-alkanes, a quantitative study of the unsymmetrical photoreaction is desirable [3,4] in order to establish whether this reaction proceeds throu& an excimer state [5-8]_ Both compounds reported by Bouas-Laurent and co-workers [I,21 have been studied in the present work, but the essential features of the reversible unsymmetric& photoreaction are covered adequately by consideration of only one compound. The present communication therefore reports a study of the photoisomerization of 1,3-bis-(9anthryl)-I, 1.3,3-tetramethyldisiloxane (I,) and the photodissociation of its photoisomer (i,), characterized by the unsymmetrical 9, I ‘; 10,4’ bonds (see scheme I). A-
Me2Si
- O-Si
Particular attention has been paid to the measurement of the quantum yields of fluorescence of i, and I,, over a range of temperature. down to 100 K in a glass forming solvent composed of methylcyclohexane and isopentane (MCI-I : IP, 1 : 3). in addition, the fluorescence spectra have been measured over the same range. The various data show that fluorescence and photocycioaddition are in competition in an extimer state which has an unsymmet~c~ conformation of the two anthracene chromophores.
2. Experimental The preparation of I, has been described [‘_I _ f, was obtained in situ, by irradiation (wavelengths greater than 340 nm) of I, in solution in evacuable
Me+
A = 9_ anthryl
fcl 446
lb
Scheme I 0 009--2614/8
1/0000-0000/S
02.50
0 North-Holland Publishing Company
CHEMICAL
Volume 78, number 3
1.5 March 1981
PHYSICS LETTERS
square cross section silica absorption celIs, degassed by several freeze-thaw cycles_ Fluorescence spectra and quantum yields were determined by methods already outlined [3] _
3. Results and discussion 3.1. Fzuort?scer2ceof
la
The fluorescence of I, shows, at room temperature, features characteristic of the 9-anthryl chromophore (localized emission, M), as well as a broad band characteristic of excimers (E) [2] _ Lowering of the temperature leads to an increase of both M and E components mitially, followed by a decrease of the E component and a continued increase of the M component. Fig. 1 shows a selection of fluorescence spectra_ There are two components to the E emission. E, with a maxiFig. 2. Temperature variation of the total fluorescence yield (@M+E), the fluorescence yield of the locahzed chromophore (on& the fluorescence yreld of the exc~mer (+) and the fluorescence quenching yield (0~) of I, 111MCH t IP.
mum near 2 1000 cm-’ and E2 at = 18 900 cm-‘. On warming from 100 K, El appears first, then El at a higher temperature_ These two excimers correspond to the two observed by Castellan et al. [2] from photodissociated Ib in MCH at 77 K. The fluorescence yield of I, varies markedly with temperature. Fig. 2 shows the variation of total yield (#hX+E), the localized fluorescence yield ($J~), the extimer emission yield (QQE)and the quenching yield QQ) obtained by difference (1 - dlM+&_
”
c i
3.2. Photochemistyy
ofi;, and I,
The quantum yield of photocycloaddition
MCH - J.Pwas measured at room temperature
of I, in
and
found to be 0.17. Excitation was at 389 nm. The quantum yield of photodissociation of 1, in MCH : IP at room temperature and irradiation at 290 nm was 0.81. 24
Fig. 1. Fluorescence spectra of Ia recorded at various temperatures in MCH : IP.
3.3. Fhorescence
of 1,
Irradiation of $, in MCH : IP gives rise to fluorescence which changes with temperature. At -100 K 447
CHEMICAL PHYSICS LETTERS
Volume 78. number 3
15 March 1981
J-
5-
3-
5-
21,1*3
22
20
18
16
CM-’ Frg. 3. Elcited-state product fluorescence spectra obtnmed by irradratron of tb in %lCH IP at 290 mu at various temperatures the emissron IS characteristic of the naphthalene chromophore and lies in the ultraviolet _At = 120 K there appears a broad exctmer band in the visible
500 cm-’ (fig. 3). Further increase of temperature leads to an mcrease of the excimer fluorescence intensity and a drop of the ultraviolet band intensity_ The excimer intensrty reaches its maximum at -225 K and then drops. The fluorescence quantum yields were determined and the naphthalene chromophore yield (@b), the extimer yield (0;) and the photoreaction yield (@k = 0.2 - &I - @ are given in fig 4. reSion at -18
3 4. AHQZYS~S Whereas the (ground-state) conformation responsible for the M emissron of I, at 100 K has unit quantum
efficiency, the (excited-state) conformation which provides the El emrssion has a non-zero internal quenching yieId. This is shown by the simultaneous rise of Q. and QB at 2130 K. With the appearance of E?, GE levels off at -160 K and the rate of increase of QQ drops. The data in figs. 1 and 2 suggest the presence of three different excited-state conformations, two of which have some overlap of the two anthracene chromophores and are popuiated via relaxation from the (ground-state) conformation which gives rise to the M 448
- ._
,_/yy----. 200
3c
T l-1: 4. Quantum yields obtained by rrradiation of Ib in MCH IP at 290 m oil is the naphthalene chromopbore fluorescence yreld, ok IS the elcimer excited-state product fluorescence yield nnd 0’R is the photoreaction yieId (0.2.-& - &)
fluorescence Relaxation to the El conformation cannot involve large movements of the chromophores because the onset of its fluorescence is at 130 K. Previous work [9] has shown that in MCH : IP, rotational reiaxatron leading to depolarization of fluorescence, takes pIace at =I20 K. so that only small rotationa motions are hkely in the relaxation from M to El conformations Transition to the second excimer conformation I&) takes place at =I45 K and the total excimer intensity remains nearly constant until -2 10 K and then it falls. Turning to the data for Ib (fig. 4), we see that quenching of the ultraviolet fluorescence begins at = 115 K and at the same time there is a growth of the excimer fluorescence (fig. 3). Between 100 and 210 K the E emission grows at the expense of the M emission and there is no quenching of the total fluorescence yield. Above 210 K however, quenching is observed and, by 230 K, the emission of the excimer has reached its maximum intensity and then it falls, similar to that observed for I,. It is natural to conclude that the quenching process is the same in each case, i.e. photocycloaddition. AdySiS of the data is simpler for Ib because of the absence of emission from the 9-anthryl chromophore. The following scheme includes only
Volume 78, number 3
CHEMICAL PHYSICS LETTERS
those reactions which are significant the interpretation of the data. 1, +hv+
and necessary
g,
(1)
kMF Ib* __, I, +hv’,
km
12-
(2)
I,*(E),
kEF
I,*(E)---+
for
(3)
Ia + kv”,
(4)
(-9 The quantum efficiency of photocycloaddiiion from the excimer (OR) is the ratio of the rate of photocycloaddition to the rate of disappearance of the extimer. It follows from (l)-(5), using steady-state conditions,
that
\ve assume that kR is temperature k,
dependent,
= A exp(-AE?/RT),
(7)
so that
6,’ -
1 = (k&A)
exp(Ai?/RT).
The data are plotted
in the Arrhenius
(8) form (8) in
15 March 1981
fig. 5. The broken line is a least-squares fit and provides kE/A = 2.5 X 10s7 and AE* = 79 kal mol-I_ Next we treat the data in fig. 2 usmg the approximations of Chandross and Dempster [lo], i.e. we have plotted lr1(1~/1~ - 1) against l/T_ Here Irnm is the excimer intensity at 210 K and IT is the intensity of the excimer emission at temperature T (210 < T< 298 K). The plot is shown in fig. 5 along with the least-squares straight line. Af? = 8.6 kcal mol-’ in good agreement with the value obtained previously (within experimental errors)_ The direct determination of the quantum yitld of photocycloaddition of 1a gave a value of 0.17 and we see from fig. 2 that the difference between the roomtemperature @a and the intercept obtained by a smooth extrapolation of the low-temperature QR curve (broken curve) IS ~0.17. The mcreased quenching above -230 K is accounted for by photocycloaddition, which competes with fluorescence from the E, conformation. It remains to establish the likely arrangement of the two chromophores in the E, conformation_ Although this cannot be done by a direct method, it is known from the work of Hayashi et al. [ 111 that [2.2] (1,4)(9,lO)anthracenophane (II) is both fluorescent and photoreactive. It therefore has the two chromophores fuced in an unsymmetrical arrangement and a weak room-temperature fluorescence was found [ 111 which became stronger on cooling to 77 K. The maximum of the 77 K fluorescence spectrum was 570 nm (=17 500 cm-‘), which is close to the 140 K fluorescence intensity maximum of the E, extimer (fig. 3). The difference can be ascribed to the effect of the two methylene bridges in II. It seems highly likely therefore, that the conformation E2, not E, as suggested previously [2], of I,, the photoreactive conformation, has an unsymmetrical arrangement of the two anthracene chromophores, in which the planes should be essentially parallel in analogy with previous results [ 1 l] _Photoreaction proceeds from this excimer state.
Acknowledgement Fiz. 5. Arrhenius plots of the quantum efficiency of photocycloaddition from the evcimer obtained by irradiation of Ib (right-hand scale) and the quenching of the excimer fluorescence of Ia (left-hand scale).
We thank F-C. de Schryver for helpful comments and for a careful reading of the manuscript.
449
Volume
78, number
3
CHEMICAL
PHYSICS
References f l] G. Felix, R. Lapouyade, H Bows-Laurent and B, CJin, Tetrahedron Letters (1976) 2277. [ 21 A. CasteBan, J.-P_ Desvergne and H. Bows-Laurent, Nouv. J. Chim 3 (1979) 231. [3] J. Ferguson, Chem Phys. Letters 76 (1980) 398. [4] G. Jones II, ‘W-R. Bergrnark and A.M. Halpern, Chem. Phys Letters 76 (1980) 403. [5] J. Ferguson and A-W.-H. Mau, Mol. Phys. 27 (i974) 377.
450
LETTERS [6] ]7] [8J [9]
[lo] fl l]
15 March 1981
N. Boens, M de Brackeleire, J. Huybrechts and F.C. de Schryver, 2. Physik. Chem. NF 101 (1976) 417_ G. K~UPP. Ann. Chem. (1973) 844. W-R. Begmark, G. Jones If, T E. Reinhardt and A M. Halpern, J. Am. Chem. Sot. 100 (1978) 6665. J. Ferguson, A.W.-H. Mau and P.O. Whimp, J. Am. Chem Sot lOI(1979) 2370 E A. Chandross and C J. Dempster, J. Am. Chem. Sot. 92 (1970) 3.568. T. Hayashr, N. Mataga, Y. Sakata, S. Misumi, M_ Monta and J. Tanaka, J. Am. Chem. Sot. 98 (1976) 5910.