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High pressure studies of isoquinoline luminescence in polymeric media
CHEMICAL PHYSICS LETTERS
VoIume 59, number 3
HIGH PRESSURE AX
ROLLINSON,
STUDJES OF ISOQUINOLINE G.B. SCHUSTER
LUMINESCENCE
1 December 1978
IN POLYMERIC
MEDIA *
and H.G. DRICKAMER
School of Gbmkal Sciences and Matetiak Resezt& Urbana, Illinois 6180 I, USA
Laboratory. U.nisetsi@ of Ihois
at Urbana-champa@.
Received 8 August 1978 The fluorescence peak of isoquinoline increases stro_nglyin intensity with increasing pressure in polymethylmethacrylate (PBIMA) and in polyisobutylene (PIB). In polyvinylalcohol the intensity decreases as the pressure increases. In all three cases the emission peak shifts to lower energy as the pressure increases_ The results are consistent with the hypothesis that the z-z* state lies higher in energy than the n-2-r* state in non-hydrogen bonding soIvenrs, but that the situation is reversed in hydrogen bonding solvents_
Tne effect of pressure (to 115 kbar) has been studied on the emission spectra of isoquinoline dissolved in polymethyfmethacrylate (EMMA) (0.03 m01/2) and polyisobutylene (E’IB) (O_OSmoI/!2) as well as in polyvinylalcohol (PVA) (O_O47 moI/!2). The luminescence of isoquinoline has been studied frequently over the past twenty years. Since the discovery in 1956 of the unusual solvent influence on fluorescence and phosphorescence, studies have been made investigating the effects of temperature [ l] and a number of solvents [%4] . Vapor phase studies have aIso been made [S] _while the luminescent behavior of isoquinoline is weIl documented, the theoretical aspects - in particular, state ordering and the intersystern crossing mechanism - stiII have not been satisfactorlIy been expiained. The problem of state ordering concerns relative positions and assignments of the fust and second excited singlet states (S1 and Sz). The most widely held theory [2,3,5] is that the lowest excited singlet (S1) is mr* in character when in hydrocarbon media, but W# when hydrogen bonded. This proposed system is consistent with calcuhztions by EI-Sayed, in which he shows that intersystem crossing from xm* 1 to nn*3 proceeds ICKIO *This work was supported in part by the Department of Energy under Contract EY-76-C-02-1198 and in part by the Donors of the Petroleum Research Fund. administeredby the American ChemicalSociety.
times faster than from the TX* 1 state [6 ] . When hydrogen bonding occurs, the nn’ singlet is believed to be shifted to an enera high enough to separate it completely from the lowest rrn* singlet region. The difficulty lies in experimental verification of these hypotheses. While studies of isoquinoline in crystalIine nsphthaIene have identified the lowest excited singlet as nn-*, tests in crystalline durene indicate that S, is ZZT*[7] _ It appears certain, however, that the singlet mr* and ~17~*states are close enough in nonhydrogen bonding media to aIlow strong coupling interaction [3,8--101. lsoquinoline (Aldrich, 97%) was purified by preparative gas. Chromatography on a Varian Aerograph Model A-90-P gas chromatograph. Separation was accomplished using a 6’ X I/4” 20% SE-30 on Chromasorb W column_ The contaminants appeared as low shoulders on either side of the pure isoquinoline fraction. In order to prevent any contamination, only the middie portion of the isoquinoline fraction was collected. The collection flask was kept seaIed to keep out moisture when not in use. l’he collected isoquinoline fraction was father purified by bulb to bulb distillation under high vacuum. The fmaI product was dissolved in a spectral grade hydrocarbon solvent and stored in the dark under a dry nitrogen atmosphere_ ‘Ihe methods of preparing and doping the polymers, as weII as the high pressure huninescence techniques znd 559
CHEMICAL PHYSICS LETTERS
Voiume 59. number 3
methods of data processing have been descriied
1 December 1978
else-
where [ii--161.
PM&Q2
Peak kcations and ir&egrated intensities were ob-
tained for both the flllorescence and phosphorescence Peaks in PMMA and PIB. Phosphorescence lifetimes were also measmed, In PVA the intensities of the fluorescence and the emission from the hydrogen bonded species were studied. The phosphorescence studies contained only limited information_ At low pressures there is significant OXygen quenc&ng but this effect disappears by 20 kbar [ 171. At higher pressures in PIB the intensity decreased with increasing pressure (by a factor of z 8 fiOlll20100 kbar) and the Sfetime decreased by about the same factor_ In PMMA the intensity appeared to increase to 40 kbar and then decreased by a factor of z 2 to 110 kbar; again the lifetime decreased in the same purportion. The increase in intensity from 2040 kbar (a factor of 2-2S) 2ppe2rs red but is difficult to interpret_ in neither solvent did the phosphorescence peak exhibit significant shifts.
The major point of interest in this work is the large increase in intensity of the fluorescence emission with
lO0
PIB I
-
5 2zo ’ 0
I 20
I 40
I 60
1 80
I 100
I 1 120
Pressure (kbarf Fig_2. !&if: in energywith prewe - fiuoreseencepezk of isoquinotie in po&metbybnetbauWate (PM-MA)and in poIyisobutylene(PIB).
increase in pressure in PMMA and PIB, as shown in fig_ 1_ This was accompanied by a red-shift of the peak which accelerated with increasing pressure (see fg_ 2). WhiIe the two media showed qualitatively similar effects, the intensity increase was much larger in PMMA, arzd above z 70 kbar the intensity leveled, and appeared to decrease slightly at the highest pressures. In PIB the intensity increase was monotonic_ As shown in table 1, the emission intensity increased with decreasing temperature at all pressures, but the temperature coefficient decreased signii?cantiy with increasing pressure_ This effect was markedly stronger in PMMA than in PIB. in neither solvent did the pea!! half width change significantly with pressure. these results can best be explained by the assumption that the Iowest fying excitation at one atmosphere has nzr* character with a R-T* level above. In general,
Pressure uctxr)
Fig. 1. ReIatke fiuorescenceintensityversuspressure- is+ quinoIinein polymet&lmetbaery~te (PMXA) and in polyisobutylene@ES).
560
n--n* excitatiocs have higher transition moments than do n-s*, and tend to decrease in energy more rapidly with incre2sing pressure. l%ere is thus an increasing li--ll* character of the en&&on tith increasing pressure as the energy difference between the states de-
Vohlme 59, number 3
CHEhfICAL PHYSICS LETTERS
TabIe 1 tow temperature intensity ratios (I 30 K/300 K)
Medium PhfMA
fluorescence
24 kbar 60 kbar 100 kbar
7.6 3.6 2.2
phosphoresoznoe 5.9 3.7 3.2
7.8
10.0
PIB 20 kbar 60 kbar
5.6
6.0
95 kbar
4.0
10.0
fluorescence
PVA
hydrogen bonded emission
20 kbar 60 kbar
1.9
5.0
1.6
3.4
ICO kbar
1.3
1.8
creases. This could be caused either by increasing mixing of the states or by increased thermal occupation of the upper (7~~-v*) level. In PMMA the n--n* to n--B* transfer appears to be complete by * 70 kbar and intensity changes at higher pressures are dominated by changes in the radiationless deactivation rate. In PIB there is apparently a larger initial spacing between the levels as rhe increase in intensity continues to the highest available pressures_ The decrease in temperature coefficient of intensity with increasing pressure, larger in PUMA
than in PIB, is consistent
with the above ex-
planation. In contrast to the above behavior, tie fluorescence peak in PVA decreased in intensity by a factor of five in 115 kbar, while exhibiting a red-shift of e 1000 a-l. No phosphorescence was observed, but there was a
1 December 1978
broad peak located near 24 K at one atmosphere which $&ted red and decreased in intensity by a factor of almost two in the pressure range covered. ‘Ihis was probably due to the hydrogen bonded emission. but excimer emission cannot be excluded_ The behavior of the fluorescence peak is consistent with the hypothesis that the 5i--5f* transition lies lower iban the nTiF in hydrogen bonding solvents.
References
111J-R_ Huber, PI
M. Mahaney and J-V. sforris, Chem. Phys. 16 <19X) 329. M-F. Anton and W.R. hfoomaw, J. Chem. Phys. 66 (1977)
1808.
[31 Y-H. Li and EC_ L&s, Cbem. Phys. Letters 9 (1971) 279. 141 V-L. Ermolaev and IP. Kotlyar, Opt. Spectry. 9 (1960)
183. 151G. Fischer and R. Naaman, Chem. Phys. 12 (1976) 367_ 161&f-A_El-Sayed, I_ Chem. phys. 38 (1963) 2834. Al). Jordan, PhD. Thesis, University of Sidney (1970).
_E C. Lim and JhI.H. Yu, J. Chem. Phys. 45 (1966) 4742.
191 E-C. Lim and J_hfH. Yu,3. Chem. Phys. 47 (1967) 3270. [IO] S-L. hfadej, S. Okajima and EC. Lim, J. Chem. PAYS. 65 (1976) 1219. [ 1 f] W.D. Drotning and H-G. Drickamer, Phys. Rev. B13 (1976) 4558. 1121 CE. Tyner and H-G. Drickamer, J. Chem. Phys. 67 (1977) 4103. [ 131 D.I. Klick, K.W. Bieg and H.G. Drickamer, Phyn Rev. B16 (1977) 4599. [14] DJ. Mitchell, G.B. Schuster and H.G. Drickamer, J. Am. Chem. See- 99 (1977) 1145. [ 15J DJ. Mitch&f. H-G_ Drickamer and G.B. Schuster, 3. Am. Chem_ Sot- 99 (1977) 7489_ [ 161 D.J. Mitchell, H.G. Drickamer and G.B. Schuster, J. Chem. Phys. 67 (1977) 4832s [ 171 BA_ Baldwin and H-W_ Offen, J- Chem. Phys. 49 (1968) 2933.
561
VoIume 59, number 3
HIGH PRESSURE AX
ROLLINSON,
STUDJES OF ISOQUINOLINE G.B. SCHUSTER
LUMINESCENCE
1 December 1978
IN POLYMERIC
MEDIA *
and H.G. DRICKAMER
School of Gbmkal Sciences and Matetiak Resezt& Urbana, Illinois 6180 I, USA
Laboratory. U.nisetsi@ of Ihois
at Urbana-champa@.
Received 8 August 1978 The fluorescence peak of isoquinoline increases stro_nglyin intensity with increasing pressure in polymethylmethacrylate (PBIMA) and in polyisobutylene (PIB). In polyvinylalcohol the intensity decreases as the pressure increases. In all three cases the emission peak shifts to lower energy as the pressure increases_ The results are consistent with the hypothesis that the z-z* state lies higher in energy than the n-2-r* state in non-hydrogen bonding soIvenrs, but that the situation is reversed in hydrogen bonding solvents_
Tne effect of pressure (to 115 kbar) has been studied on the emission spectra of isoquinoline dissolved in polymethyfmethacrylate (EMMA) (0.03 m01/2) and polyisobutylene (E’IB) (O_OSmoI/!2) as well as in polyvinylalcohol (PVA) (O_O47 moI/!2). The luminescence of isoquinoline has been studied frequently over the past twenty years. Since the discovery in 1956 of the unusual solvent influence on fluorescence and phosphorescence, studies have been made investigating the effects of temperature [ l] and a number of solvents [%4] . Vapor phase studies have aIso been made [S] _while the luminescent behavior of isoquinoline is weIl documented, the theoretical aspects - in particular, state ordering and the intersystern crossing mechanism - stiII have not been satisfactorlIy been expiained. The problem of state ordering concerns relative positions and assignments of the fust and second excited singlet states (S1 and Sz). The most widely held theory [2,3,5] is that the lowest excited singlet (S1) is mr* in character when in hydrocarbon media, but W# when hydrogen bonded. This proposed system is consistent with calcuhztions by EI-Sayed, in which he shows that intersystem crossing from xm* 1 to nn*3 proceeds ICKIO *This work was supported in part by the Department of Energy under Contract EY-76-C-02-1198 and in part by the Donors of the Petroleum Research Fund. administeredby the American ChemicalSociety.
times faster than from the TX* 1 state [6 ] . When hydrogen bonding occurs, the nn’ singlet is believed to be shifted to an enera high enough to separate it completely from the lowest rrn* singlet region. The difficulty lies in experimental verification of these hypotheses. While studies of isoquinoline in crystalIine nsphthaIene have identified the lowest excited singlet as nn-*, tests in crystalline durene indicate that S, is ZZT*[7] _ It appears certain, however, that the singlet mr* and ~17~*states are close enough in nonhydrogen bonding media to aIlow strong coupling interaction [3,8--101. lsoquinoline (Aldrich, 97%) was purified by preparative gas. Chromatography on a Varian Aerograph Model A-90-P gas chromatograph. Separation was accomplished using a 6’ X I/4” 20% SE-30 on Chromasorb W column_ The contaminants appeared as low shoulders on either side of the pure isoquinoline fraction. In order to prevent any contamination, only the middie portion of the isoquinoline fraction was collected. The collection flask was kept seaIed to keep out moisture when not in use. l’he collected isoquinoline fraction was father purified by bulb to bulb distillation under high vacuum. The fmaI product was dissolved in a spectral grade hydrocarbon solvent and stored in the dark under a dry nitrogen atmosphere_ ‘Ihe methods of preparing and doping the polymers, as weII as the high pressure huninescence techniques znd 559
CHEMICAL PHYSICS LETTERS
Voiume 59. number 3
methods of data processing have been descriied
1 December 1978
else-
where [ii--161.
PM&Q2
Peak kcations and ir&egrated intensities were ob-
tained for both the flllorescence and phosphorescence Peaks in PMMA and PIB. Phosphorescence lifetimes were also measmed, In PVA the intensities of the fluorescence and the emission from the hydrogen bonded species were studied. The phosphorescence studies contained only limited information_ At low pressures there is significant OXygen quenc&ng but this effect disappears by 20 kbar [ 171. At higher pressures in PIB the intensity decreased with increasing pressure (by a factor of z 8 fiOlll20100 kbar) and the Sfetime decreased by about the same factor_ In PMMA the intensity appeared to increase to 40 kbar and then decreased by a factor of z 2 to 110 kbar; again the lifetime decreased in the same purportion. The increase in intensity from 2040 kbar (a factor of 2-2S) 2ppe2rs red but is difficult to interpret_ in neither solvent did the phosphorescence peak exhibit significant shifts.
The major point of interest in this work is the large increase in intensity of the fluorescence emission with
lO0
PIB I
-
5 2zo ’ 0
I 20
I 40
I 60
1 80
I 100
I 1 120
Pressure (kbarf Fig_2. !&if: in energywith prewe - fiuoreseencepezk of isoquinotie in po&metbybnetbauWate (PM-MA)and in poIyisobutylene(PIB).
increase in pressure in PMMA and PIB, as shown in fig_ 1_ This was accompanied by a red-shift of the peak which accelerated with increasing pressure (see fg_ 2). WhiIe the two media showed qualitatively similar effects, the intensity increase was much larger in PMMA, arzd above z 70 kbar the intensity leveled, and appeared to decrease slightly at the highest pressures. In PIB the intensity increase was monotonic_ As shown in table 1, the emission intensity increased with decreasing temperature at all pressures, but the temperature coefficient decreased signii?cantiy with increasing pressure_ This effect was markedly stronger in PMMA than in PIB. in neither solvent did the pea!! half width change significantly with pressure. these results can best be explained by the assumption that the Iowest fying excitation at one atmosphere has nzr* character with a R-T* level above. In general,
Pressure uctxr)
Fig. 1. ReIatke fiuorescenceintensityversuspressure- is+ quinoIinein polymet&lmetbaery~te (PMXA) and in polyisobutylene@ES).
560
n--n* excitatiocs have higher transition moments than do n-s*, and tend to decrease in energy more rapidly with incre2sing pressure. l%ere is thus an increasing li--ll* character of the en&&on tith increasing pressure as the energy difference between the states de-
Vohlme 59, number 3
CHEhfICAL PHYSICS LETTERS
TabIe 1 tow temperature intensity ratios (I 30 K/300 K)
Medium PhfMA
fluorescence
24 kbar 60 kbar 100 kbar
7.6 3.6 2.2
phosphoresoznoe 5.9 3.7 3.2
7.8
10.0
PIB 20 kbar 60 kbar
5.6
6.0
95 kbar
4.0
10.0
fluorescence
PVA
hydrogen bonded emission
20 kbar 60 kbar
1.9
5.0
1.6
3.4
ICO kbar
1.3
1.8
creases. This could be caused either by increasing mixing of the states or by increased thermal occupation of the upper (7~~-v*) level. In PMMA the n--n* to n--B* transfer appears to be complete by * 70 kbar and intensity changes at higher pressures are dominated by changes in the radiationless deactivation rate. In PIB there is apparently a larger initial spacing between the levels as rhe increase in intensity continues to the highest available pressures_ The decrease in temperature coefficient of intensity with increasing pressure, larger in PUMA
than in PIB, is consistent
with the above ex-
planation. In contrast to the above behavior, tie fluorescence peak in PVA decreased in intensity by a factor of five in 115 kbar, while exhibiting a red-shift of e 1000 a-l. No phosphorescence was observed, but there was a
1 December 1978
broad peak located near 24 K at one atmosphere which $&ted red and decreased in intensity by a factor of almost two in the pressure range covered. ‘Ihis was probably due to the hydrogen bonded emission. but excimer emission cannot be excluded_ The behavior of the fluorescence peak is consistent with the hypothesis that the 5i--5f* transition lies lower iban the nTiF in hydrogen bonding solvents.
References
111J-R_ Huber, PI
M. Mahaney and J-V. sforris, Chem. Phys. 16 <19X) 329. M-F. Anton and W.R. hfoomaw, J. Chem. Phys. 66 (1977)
1808.
[31 Y-H. Li and EC_ L&s, Cbem. Phys. Letters 9 (1971) 279. 141 V-L. Ermolaev and IP. Kotlyar, Opt. Spectry. 9 (1960)
183. 151G. Fischer and R. Naaman, Chem. Phys. 12 (1976) 367_ 161&f-A_El-Sayed, I_ Chem. phys. 38 (1963) 2834. Al). Jordan, PhD. Thesis, University of Sidney (1970).
_E C. Lim and JhI.H. Yu, J. Chem. Phys. 45 (1966) 4742.
191 E-C. Lim and J_hfH. Yu,3. Chem. Phys. 47 (1967) 3270. [IO] S-L. hfadej, S. Okajima and EC. Lim, J. Chem. PAYS. 65 (1976) 1219. [ 1 f] W.D. Drotning and H-G. Drickamer, Phys. Rev. B13 (1976) 4558. 1121 CE. Tyner and H-G. Drickamer, J. Chem. Phys. 67 (1977) 4103. [ 131 D.I. Klick, K.W. Bieg and H.G. Drickamer, Phyn Rev. B16 (1977) 4599. [14] DJ. Mitchell, G.B. Schuster and H.G. Drickamer, J. Am. Chem. See- 99 (1977) 1145. [ 15J DJ. Mitch&f. H-G_ Drickamer and G.B. Schuster, 3. Am. Chem_ Sot- 99 (1977) 7489_ [ 161 D.J. Mitchell, H.G. Drickamer and G.B. Schuster, J. Chem. Phys. 67 (1977) 4832s [ 171 BA_ Baldwin and H-W_ Offen, J- Chem. Phys. 49 (1968) 2933.
561