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Effect of a matrix of polymethylmethacrylate on triplet-triplet transitions of anthracene, phenanthrene, and like heterocycles
JOURNAL OF MOLECULAR SPECTROSCOPY
132,238-241 (1988)
Effect of a Matrix of Polymethylmethacrylate on Triplet-Triplet Transitions of Anthracene, Phenanthrene, and Like Heterocycles J. I. DEL BARRIO, J. R. REBATO, AND F. M. G.-TABLAS Departamentode Quimica, hive&dad Aut6noma de Madrid, Cantoblanco,28049 Madrid, Spain Transient triplet-triplet absorption spectra of anthracene, phenanthrene, acridine, phenazine, and phenanthridine have been observed by flash photolysis in matrices of polymetbylmethacrylate at room temperature. The results are compared with similar transitions in the vapor phase. 0 1988 Academic Press, Inc. INTRODUCTION
The triplet-triplet transitions of aromatic and heterocyclic molecules have been studied by flash photolysis mainly in solutions of polar and nonpolar solvents (1-5). Recently, we have studied (6, 7) these transitions in the vapor phase and systematically analyzed the solvatochromic shifts of a series of polar and nonpolar solvents. From these measurements and using McRae’s (8) reaction field model developed by Amos and Burrows (9), we have been able to estimate the polarizability difference between the states involved in the transitions. Although some triplet-triplet spectra have been observed in EPA at 77 K, very few have been reported in solutions of polymethylmethacrylate (PMM) at room temperature. In the present work we report the absorption spectra of the above compounds in solutions of polymethylmethacrylate at room temperature. EXPERIMENTAL
DETAILS
The classical flash-photolysis technique was used to record the triplet-triplet absorption spectra with a Hilger medium spectroscope on Kodak Panchromatic Plus X film plates. The photolytic lamp for the PMM experiments was 14 cm long, filled with 30 Torr of Ar, and produced flashes with a width of 30 ps, discharging 500 J (10 PF at 10 kV). In the gas-phase experiments, the photolytic lamp, 77 cm long and 1.2 cm i.d., produced 40-ps half-life flashes, discharging the same energy. The spectroscopic lamp used in all cases was 7 cm long with a flash duration of 18 ~LSat half-width. The methylmethacrylate monomer was supplied by B.D.H. Chemicals Ltd. and was used without further purification. All the solutes were supplied by Merck and were purified by sublimation under vacuum. The solutions were repeatedly degassed by freeze and pump cycles. The degassed solution was polymerized in a stove at 80°C for several hours. The resulting cylindrical matrices were mechanized in 5-cm-long cylinders of 2-cm diameter. The ends were polished until a good optical transparency was achieved. Other details of the experimental setup have been reported elsewhere (6). 0022-2852/88 $3.00 Copyright 0 1988 by Academic Press, Inc. All rights of reproduction in any form reserved.
238
239
EFFECT OF PMM MATRIX ON T-T TRANSITIONS RESULTS AND DISCUSSION
Figure 1 shows the transient triplet-triplet absorption spectra of anthracene, phenanthrene, a&dine, and phenazine in the vapor phase and in solutions of polymethylmethacrylate at room temperature. In the acrylic plastic, all maxima are red shifted when compared with transitions in the vapor phase. We have assumed that all the observed absorption bands correspond to transitions that originate from the lowest triplet state, T, . It is well known (10-12) that for this kind of molecule, several triplet states lie below the first excited singlet, S, , and that the population of the triplet manifold occurs through the intersystem crossing from S, to the closest triplet state, c. The process Tj * T, is very rapid, especially when the q is a 3(1r,?r*), as it seems to be the case for acridine (I I). Therefore, the assumption that the observed absorptions start from Tl seems quite reasonable.
BI VAPOR PMM
PHASE
LOO
360
LLO Xlnml
Xtnml
480
52C
C --
360
VAPOR
LB0
LOO X(nml
PHASE
- - VAPOR
360
uxl
LLO
PHASE
480 Mnml
FIG 1. Transient triplet-triplet absorption spectra in vapor phase and in polymethylmethacrylate of (A) anthracene, (B) phenanthrene, (C) a&dine, and (D) phenazine.
240
DEL BARRIO, REBATO, AND G.-TABLAS TABLE I Position
(p) and Width at Half-Height (w) of q + T, Transitions in PMM
PH-DINE ACRIDINE
415
15
440
24
486
--
PHENAZINE
417
15
441
33
472
--
Table I shows the positions and widths at half-height of the maxima observed in PMM. Table II presents the corresponding solvatochromic shifts with respect to the same transitions in the vapor phase. The sign and magnitude of the solvent shift are determined by the polarity and hydrogen-bonding ability of the solvent, the character of the transition, (n, ?r*) or (x, ?r*), and the type of interaction between the solvent and solute. The experimental red shifts observed in all cases seem to indicate that all the transitions we have studied correspond to the type A + ?r*, the same as suggested by our previous results on the solvatochromic shifts in a series of alcohols and nonpolar solvents (6, 7). This conclusion is in accordance with the well-known fact that the lowest triplet state of all the molecules studied is a 3(r, ?r*) state. The values obtained in PMM show displacements similar to those observed in nonpolar solvents (6, 7) such as n-heptane and benzene. In the case of phenanthrene, the PMM shifts are somewhat larger than those in the usual polar and nonpolar liquid solvents. The red shift occurs because the upper state of the triplet-triplet transition is more strongly solvated than the lower one. This conclusion agrees with the fact that the energy of T1 (r, 7~*)of acridine seems to be solvent independent (13). This could also be the case for the other compounds studied and we can conclude that the solvation energy of the T1 state is between 1000 and 2000 cm-’ smaller than the solvation energy of the upper triplet states. TABLE II Solvatochromic Shifts in PMM: AZ&-T= Cvapor pha~- ip~~ (in cm-‘)
ANIHRACENE
1467
1223
-___
PHENANIWUENE
____
1751
1716
ACRIDINE
2151
1527
-___
FHBWINE
1523
993
-DINE
1320
1774
-_-_
EFFECT
OF PMM MATRIX
ON T-T
TRANSITIONS
241
ACKNOWLEDGMENT We thank the CAYCIT for supporting this research. RECEIVED:
February 29, 1988 REFERENCES
1. G. PORTERAND M. WINDSOR,Discuss. Faraday Sot. 17, 178-186 (1954). 2. G. PORTERAND M. WINDSOR,Proc. R. Sot. A 245,238-258 (1958). 3. S. P. MCGLYNN, T. AZUMI, ANDM. K~NOSHITA, “Molecular Spectroscopy of the Triplet State,” PrenticeHall, Englewood Cliffs, NJ, 1969. 4. F. WILKINSON,“Organic Molecular Photophysics” (J. Birks, Ed.), Wiley, New York, 1975. 5. B. STEVENSAND M. THOMAS, Chem. Phys. Lett. 1,535-543 (1968). 6. J. I. DEL BARRIO, J. R. REBATO, AND F. M. G.-TABLAS, Chem. Phys. Lett. 114,397-400 (1985). 7. J. I. DEL BARRIO, J. R. REBATO, AND F. M. G.-TABLAS, J. Phys. Chem. 90,281 l-2812 (1986). 8. E. G. MCRAE, J. Phys. Chem. 61,562-572 (1957). 9. A. T. AMOS ANDB. L. BURROWS,in “Advances in Quantum Chemistry” (P.-O. Liiwdin, Ed.), Academic Press, New York, 1973. 10. L. G~~DMANNANDR. W. HARRELL,J. Chem. Phys. 30, 1131-1138 (1959). Il. Y. HIRATA AND I. TANAKA, Chem. Phys. Lett. 41,336-338 (1976). 12. J. L. BAPTISTA,S. J. FORMOSINHO,AND M. DE F. LEITAO, Chem. Phys. 28,425-432 (1978). 13. N. PERIASAMY,Chem. Phys. Lett. 99,322-325 (1983).
132,238-241 (1988)
Effect of a Matrix of Polymethylmethacrylate on Triplet-Triplet Transitions of Anthracene, Phenanthrene, and Like Heterocycles J. I. DEL BARRIO, J. R. REBATO, AND F. M. G.-TABLAS Departamentode Quimica, hive&dad Aut6noma de Madrid, Cantoblanco,28049 Madrid, Spain Transient triplet-triplet absorption spectra of anthracene, phenanthrene, acridine, phenazine, and phenanthridine have been observed by flash photolysis in matrices of polymetbylmethacrylate at room temperature. The results are compared with similar transitions in the vapor phase. 0 1988 Academic Press, Inc. INTRODUCTION
The triplet-triplet transitions of aromatic and heterocyclic molecules have been studied by flash photolysis mainly in solutions of polar and nonpolar solvents (1-5). Recently, we have studied (6, 7) these transitions in the vapor phase and systematically analyzed the solvatochromic shifts of a series of polar and nonpolar solvents. From these measurements and using McRae’s (8) reaction field model developed by Amos and Burrows (9), we have been able to estimate the polarizability difference between the states involved in the transitions. Although some triplet-triplet spectra have been observed in EPA at 77 K, very few have been reported in solutions of polymethylmethacrylate (PMM) at room temperature. In the present work we report the absorption spectra of the above compounds in solutions of polymethylmethacrylate at room temperature. EXPERIMENTAL
DETAILS
The classical flash-photolysis technique was used to record the triplet-triplet absorption spectra with a Hilger medium spectroscope on Kodak Panchromatic Plus X film plates. The photolytic lamp for the PMM experiments was 14 cm long, filled with 30 Torr of Ar, and produced flashes with a width of 30 ps, discharging 500 J (10 PF at 10 kV). In the gas-phase experiments, the photolytic lamp, 77 cm long and 1.2 cm i.d., produced 40-ps half-life flashes, discharging the same energy. The spectroscopic lamp used in all cases was 7 cm long with a flash duration of 18 ~LSat half-width. The methylmethacrylate monomer was supplied by B.D.H. Chemicals Ltd. and was used without further purification. All the solutes were supplied by Merck and were purified by sublimation under vacuum. The solutions were repeatedly degassed by freeze and pump cycles. The degassed solution was polymerized in a stove at 80°C for several hours. The resulting cylindrical matrices were mechanized in 5-cm-long cylinders of 2-cm diameter. The ends were polished until a good optical transparency was achieved. Other details of the experimental setup have been reported elsewhere (6). 0022-2852/88 $3.00 Copyright 0 1988 by Academic Press, Inc. All rights of reproduction in any form reserved.
238
239
EFFECT OF PMM MATRIX ON T-T TRANSITIONS RESULTS AND DISCUSSION
Figure 1 shows the transient triplet-triplet absorption spectra of anthracene, phenanthrene, a&dine, and phenazine in the vapor phase and in solutions of polymethylmethacrylate at room temperature. In the acrylic plastic, all maxima are red shifted when compared with transitions in the vapor phase. We have assumed that all the observed absorption bands correspond to transitions that originate from the lowest triplet state, T, . It is well known (10-12) that for this kind of molecule, several triplet states lie below the first excited singlet, S, , and that the population of the triplet manifold occurs through the intersystem crossing from S, to the closest triplet state, c. The process Tj * T, is very rapid, especially when the q is a 3(1r,?r*), as it seems to be the case for acridine (I I). Therefore, the assumption that the observed absorptions start from Tl seems quite reasonable.
BI VAPOR PMM
PHASE
LOO
360
LLO Xlnml
Xtnml
480
52C
C --
360
VAPOR
LB0
LOO X(nml
PHASE
- - VAPOR
360
uxl
LLO
PHASE
480 Mnml
FIG 1. Transient triplet-triplet absorption spectra in vapor phase and in polymethylmethacrylate of (A) anthracene, (B) phenanthrene, (C) a&dine, and (D) phenazine.
240
DEL BARRIO, REBATO, AND G.-TABLAS TABLE I Position
(p) and Width at Half-Height (w) of q + T, Transitions in PMM
PH-DINE ACRIDINE
415
15
440
24
486
--
PHENAZINE
417
15
441
33
472
--
Table I shows the positions and widths at half-height of the maxima observed in PMM. Table II presents the corresponding solvatochromic shifts with respect to the same transitions in the vapor phase. The sign and magnitude of the solvent shift are determined by the polarity and hydrogen-bonding ability of the solvent, the character of the transition, (n, ?r*) or (x, ?r*), and the type of interaction between the solvent and solute. The experimental red shifts observed in all cases seem to indicate that all the transitions we have studied correspond to the type A + ?r*, the same as suggested by our previous results on the solvatochromic shifts in a series of alcohols and nonpolar solvents (6, 7). This conclusion is in accordance with the well-known fact that the lowest triplet state of all the molecules studied is a 3(r, ?r*) state. The values obtained in PMM show displacements similar to those observed in nonpolar solvents (6, 7) such as n-heptane and benzene. In the case of phenanthrene, the PMM shifts are somewhat larger than those in the usual polar and nonpolar liquid solvents. The red shift occurs because the upper state of the triplet-triplet transition is more strongly solvated than the lower one. This conclusion agrees with the fact that the energy of T1 (r, 7~*)of acridine seems to be solvent independent (13). This could also be the case for the other compounds studied and we can conclude that the solvation energy of the T1 state is between 1000 and 2000 cm-’ smaller than the solvation energy of the upper triplet states. TABLE II Solvatochromic Shifts in PMM: AZ&-T= Cvapor pha~- ip~~ (in cm-‘)
ANIHRACENE
1467
1223
-___
PHENANIWUENE
____
1751
1716
ACRIDINE
2151
1527
-___
FHBWINE
1523
993
-DINE
1320
1774
-_-_
EFFECT
OF PMM MATRIX
ON T-T
TRANSITIONS
241
ACKNOWLEDGMENT We thank the CAYCIT for supporting this research. RECEIVED:
February 29, 1988 REFERENCES
1. G. PORTERAND M. WINDSOR,Discuss. Faraday Sot. 17, 178-186 (1954). 2. G. PORTERAND M. WINDSOR,Proc. R. Sot. A 245,238-258 (1958). 3. S. P. MCGLYNN, T. AZUMI, ANDM. K~NOSHITA, “Molecular Spectroscopy of the Triplet State,” PrenticeHall, Englewood Cliffs, NJ, 1969. 4. F. WILKINSON,“Organic Molecular Photophysics” (J. Birks, Ed.), Wiley, New York, 1975. 5. B. STEVENSAND M. THOMAS, Chem. Phys. Lett. 1,535-543 (1968). 6. J. I. DEL BARRIO, J. R. REBATO, AND F. M. G.-TABLAS, Chem. Phys. Lett. 114,397-400 (1985). 7. J. I. DEL BARRIO, J. R. REBATO, AND F. M. G.-TABLAS, J. Phys. Chem. 90,281 l-2812 (1986). 8. E. G. MCRAE, J. Phys. Chem. 61,562-572 (1957). 9. A. T. AMOS ANDB. L. BURROWS,in “Advances in Quantum Chemistry” (P.-O. Liiwdin, Ed.), Academic Press, New York, 1973. 10. L. G~~DMANNANDR. W. HARRELL,J. Chem. Phys. 30, 1131-1138 (1959). Il. Y. HIRATA AND I. TANAKA, Chem. Phys. Lett. 41,336-338 (1976). 12. J. L. BAPTISTA,S. J. FORMOSINHO,AND M. DE F. LEITAO, Chem. Phys. 28,425-432 (1978). 13. N. PERIASAMY,Chem. Phys. Lett. 99,322-325 (1983).