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Solution fluorescence spectrum of highly purified fluorene
CHERIICALPHYSICSLETTERS
Volume 5. number 2
SOLUTION
FLUORESCENCE
Chentislq~
Division,
SPECTRUM
OF
HIGHLY
D. L. HORROCKS and W. G. BROWN Argonne Nutional Laboratory, Argonne,
Received
27 January
1
PURIFIED
&larch 2970
FLUORENE
ILlinois 60439,
+
USA
1970
Highly purified specimens of fluorene exhibited fluorescence with maximum at 31 700 cm-l without significant fine structure at lower frequencies in dilute solutions. At high concentratipns excimer emission occurs and is seen, at low temperatures, as a new maximum at 27 000 cm with resultant implications as to the excited state geometry.
The fluorescence spectrum of fluorene, in crystalline form and in solution, has been extensively investigated with frequently discordant results usually attributed to impurities. Recent descriptions [l-3] of the solution spectrum tend toward agreement on a strong peak at about 33 100 cm-l and two or more weaker peaks or inflections at lower frequencies. There are contrary reports [4,5] to the effect that the 33.100 cm-l is either absent or relatively weak and there are differences in detail as to structure at lower frequencies. We observed structural detail, more prominently with increasing concentration, having a probable origin in impurities and we confirmed by gas chromatography that these samples contained a number of impurities at the parts per thousand level. Conversely, chromatographically “clean” samples exhibited a single, slightly asymmetrical band, urnax 31 700 cm-l, (fig. la), over a wide range of concentration t. At very high concentrations a new feature at 27 000 cm-l became evident which, by virtue of its temperature and concentration dependence (fig. lb and c), is clearly due to excimer format:on. Three samples of fluorene of different history exhiloited identical fluorescence characteristics. One was Eastman Kodak reagent fluorene purified by repeated treatment in heptane solution with sulfuric acid, recrystallization from heptane, zone-refining ‘c, and preparative gas chromato* Work done under the auspices of the U.S. Atomic Energy Commission. ? This result is in agreement with the published curve of Munro and co-workers 151if it is assumed that our equipment would not have resolved the minor peak shown in their curve at 32 600 cm-l.
graphy,
in that order.
The
second
was
prepared
by ihe Clemmensen reduction [7] of 9-fluorenone (Eastman Kodak, recrystallized) and was purified by recrystallization from alcohol and from benzene, fractional elution from a neutral alumina column by petroleum ether. and vacuum subIimation. The third involved a two-stage reduction of 9-fluorenone (Eastman Kodak, purified by zone refining) first to fluorenol with NaBH4 in methanol [a], and then reduction with red phosphorus and iodine [9] ** with final purification by recrystallization from acetic acid, fractiona: elution from an alumina column and vacuum sublimation. The UV absorption spectra agreed with that published by Berlman [l]. The spectra were recorded with an AmincoBowman spectrophotofluorometer with corrected spectra accessory (American Instrument Co.) on de-gassed solutions in sealed quartz tubes. Front surface observation techniques were employed and the exciting energy was varied from 40 000 to 33 000 cm-l to minimize distortion due to self absorption. As a test case, acenaphthene produced a spectrum identical in all detail with a published spectrum [II-
essential
The spectrum of fluorene in dilute solution (down TV5 x 10-4 M) was examined in cyclohexane and in toluene without appreciable solvent effect. The high concentration range was studied in toluene 3 Zone refining, although effective in remwlng anthracene, did not eliminate dihenzofuran, which elutes before fluorene on a QF-1 column at 140°, or
other impurities of unknown identity with Ionger &Ltion times than fluorene. ** This procedure gave a cleaner product than the more commonly used combiaatica of red phosphorus and
HI.
CHEMlCAL
Volume 5: number2~
(al
AT 100 T
PHYSICS
Ib)
LETTERS
AT 0%
1 March 1970
Id
AT-78%
t-
MONOMER FLUORESCENCE EXCIMER FLUORESCENCI
/
/
/cl i 000 WAVENUMBER. Fig.
1
30 000
25 OCQ 20
O(70
cm-'
Fluorescence spectra from a 0.26 AI toluene solution of highly purified fluorene for excitation with 35 700 ratiation at a) lCIO°C,b) 0°C and c) -78OC. a) is indistinguishable from the spectrum of a 5 x 10Dd Iv1soiution
1.
cm-1
at robm temperature.
(b)
(cl
AT 0%
AT -78
-C
EXCIMER FLUORESCENCE EXCIMER FLUORESCENCE
/
IONOHER
350003000025000
Fig,
2.
Fluorescence spectra
cm-’
from a 0,3_61 Id toluene solution of highly purified dibenzofuran for excitation with -A a) 100°C, b) O°C and C) --lE°C38 700 cm yxsdiation
(solubility exceeds 200 g/l). In this range the monomer fluorescence yield decreased with increasing concentration and there is possibly some self-quenching as well as excimer formation. Since dibenzofuran is a known impurity in ordinary fluorene and is reported [3-51 to have a strong fluorescence peak at about 33 000 cm’], we re-examined this spectrum on a sample purified by gas chromatography. The emission was observed as an unresolved shoulder at about 32 800 cm-l with low intensity relative to the 118
h 350003000025ooo20ooo
350003000025000 WAVENUMBER.
ONOMER JORESCENCE
maxima at 31 600 and 30 606 cm-l. It seems WIlikely therefore that this substance, as a trace impurity, could account for the high intensity of the 33 000 cm-l peak in some previously published spectra of fluorene. It is possible that the maximum of dibenzofuran at about 30 600 cm-1 may be responsible for some of the reported structure in this region. At high concentrations, the purified dibenzofuran exhibited a broad extimer emission peaking at 26 700 cm’1 easily recognizable even at room temperature (fig. 2). Berlman 19) has also detected excimer formation in dibeniofuran.
Volume 5. number 2
CHEMICAL PHYSICSLETTERS
1 hlarch 1970
Excimer formation from fluorene corroborates other evidence for planarity of the molecular
REFERENCES
skeleton in the excited singlet state. The most revealing evidence is certainly the recently discovered [lo] very high acidity associated with this state. This can only be explained by a geometry closely related to that of the anion generated by removal of a proton at the 9-position. The anion is undoubtedly planar with the remaining g-hydrogen in the equatorial plane. It is thus suggested that the excited neutral molecule will have one of the 9-hydrogens in-plane (equatorial), the other out-of-plane (axial) and loosely bound.
B. Berlman, Handbookof fluorescence spectra of aromatic molecules (Academic Press, Sew York. 1965). [Z] M. Nakamizo and Y. Kanda, Spectrocbim. Acta 19 (1963) 1235. [3) D. W. Ellis and B.S. Solomon, J. Chem. Phys. 46 (1967) 3497. [4] 0. Dann and P. Nickel, Ann. Chem. 667 (1963) 101. (51I. H. Munro, ‘I’. D. S. Hamilton, J. P. Ray and G. F. Moore, Phys. Letters 20 (1966) 356. [SJH. L. Bradlow and C.A.VanderWerf. J. Am. Chem. SOC.69 (1947j 1254. [‘i] S.W. Chaikin and W.G. Brown, J. Am. Chem. Sot. 71 (1949) 122. [8] H. F. Miller and G. B.Bacbman, J. Am. Chem. Sot. 57 (1935) 2448. [S] I. B. Berlman, private communication. [lo] E. Vander Donckt, J. NasieIsM ad P.Thiry, Cbem. Common. (1969) 1249. [l]
I.
11s
Volume 5. number 2
SOLUTION
FLUORESCENCE
Chentislq~
Division,
SPECTRUM
OF
HIGHLY
D. L. HORROCKS and W. G. BROWN Argonne Nutional Laboratory, Argonne,
Received
27 January
1
PURIFIED
&larch 2970
FLUORENE
ILlinois 60439,
+
USA
1970
Highly purified specimens of fluorene exhibited fluorescence with maximum at 31 700 cm-l without significant fine structure at lower frequencies in dilute solutions. At high concentratipns excimer emission occurs and is seen, at low temperatures, as a new maximum at 27 000 cm with resultant implications as to the excited state geometry.
The fluorescence spectrum of fluorene, in crystalline form and in solution, has been extensively investigated with frequently discordant results usually attributed to impurities. Recent descriptions [l-3] of the solution spectrum tend toward agreement on a strong peak at about 33 100 cm-l and two or more weaker peaks or inflections at lower frequencies. There are contrary reports [4,5] to the effect that the 33.100 cm-l is either absent or relatively weak and there are differences in detail as to structure at lower frequencies. We observed structural detail, more prominently with increasing concentration, having a probable origin in impurities and we confirmed by gas chromatography that these samples contained a number of impurities at the parts per thousand level. Conversely, chromatographically “clean” samples exhibited a single, slightly asymmetrical band, urnax 31 700 cm-l, (fig. la), over a wide range of concentration t. At very high concentrations a new feature at 27 000 cm-l became evident which, by virtue of its temperature and concentration dependence (fig. lb and c), is clearly due to excimer format:on. Three samples of fluorene of different history exhiloited identical fluorescence characteristics. One was Eastman Kodak reagent fluorene purified by repeated treatment in heptane solution with sulfuric acid, recrystallization from heptane, zone-refining ‘c, and preparative gas chromato* Work done under the auspices of the U.S. Atomic Energy Commission. ? This result is in agreement with the published curve of Munro and co-workers 151if it is assumed that our equipment would not have resolved the minor peak shown in their curve at 32 600 cm-l.
graphy,
in that order.
The
second
was
prepared
by ihe Clemmensen reduction [7] of 9-fluorenone (Eastman Kodak, recrystallized) and was purified by recrystallization from alcohol and from benzene, fractional elution from a neutral alumina column by petroleum ether. and vacuum subIimation. The third involved a two-stage reduction of 9-fluorenone (Eastman Kodak, purified by zone refining) first to fluorenol with NaBH4 in methanol [a], and then reduction with red phosphorus and iodine [9] ** with final purification by recrystallization from acetic acid, fractiona: elution from an alumina column and vacuum sublimation. The UV absorption spectra agreed with that published by Berlman [l]. The spectra were recorded with an AmincoBowman spectrophotofluorometer with corrected spectra accessory (American Instrument Co.) on de-gassed solutions in sealed quartz tubes. Front surface observation techniques were employed and the exciting energy was varied from 40 000 to 33 000 cm-l to minimize distortion due to self absorption. As a test case, acenaphthene produced a spectrum identical in all detail with a published spectrum [II-
essential
The spectrum of fluorene in dilute solution (down TV5 x 10-4 M) was examined in cyclohexane and in toluene without appreciable solvent effect. The high concentration range was studied in toluene 3 Zone refining, although effective in remwlng anthracene, did not eliminate dihenzofuran, which elutes before fluorene on a QF-1 column at 140°, or
other impurities of unknown identity with Ionger &Ltion times than fluorene. ** This procedure gave a cleaner product than the more commonly used combiaatica of red phosphorus and
HI.
CHEMlCAL
Volume 5: number2~
(al
AT 100 T
PHYSICS
Ib)
LETTERS
AT 0%
1 March 1970
Id
AT-78%
t-
MONOMER FLUORESCENCE EXCIMER FLUORESCENCI
/
/
/cl i 000 WAVENUMBER. Fig.
1
30 000
25 OCQ 20
O(70
cm-'
Fluorescence spectra from a 0.26 AI toluene solution of highly purified fluorene for excitation with 35 700 ratiation at a) lCIO°C,b) 0°C and c) -78OC. a) is indistinguishable from the spectrum of a 5 x 10Dd Iv1soiution
1.
cm-1
at robm temperature.
(b)
(cl
AT 0%
AT -78
-C
EXCIMER FLUORESCENCE EXCIMER FLUORESCENCE
/
IONOHER
350003000025000
Fig,
2.
Fluorescence spectra
cm-’
from a 0,3_61 Id toluene solution of highly purified dibenzofuran for excitation with -A a) 100°C, b) O°C and C) --lE°C38 700 cm yxsdiation
(solubility exceeds 200 g/l). In this range the monomer fluorescence yield decreased with increasing concentration and there is possibly some self-quenching as well as excimer formation. Since dibenzofuran is a known impurity in ordinary fluorene and is reported [3-51 to have a strong fluorescence peak at about 33 000 cm’], we re-examined this spectrum on a sample purified by gas chromatography. The emission was observed as an unresolved shoulder at about 32 800 cm-l with low intensity relative to the 118
h 350003000025ooo20ooo
350003000025000 WAVENUMBER.
ONOMER JORESCENCE
maxima at 31 600 and 30 606 cm-l. It seems WIlikely therefore that this substance, as a trace impurity, could account for the high intensity of the 33 000 cm-l peak in some previously published spectra of fluorene. It is possible that the maximum of dibenzofuran at about 30 600 cm-1 may be responsible for some of the reported structure in this region. At high concentrations, the purified dibenzofuran exhibited a broad extimer emission peaking at 26 700 cm’1 easily recognizable even at room temperature (fig. 2). Berlman 19) has also detected excimer formation in dibeniofuran.
Volume 5. number 2
CHEMICAL PHYSICSLETTERS
1 hlarch 1970
Excimer formation from fluorene corroborates other evidence for planarity of the molecular
REFERENCES
skeleton in the excited singlet state. The most revealing evidence is certainly the recently discovered [lo] very high acidity associated with this state. This can only be explained by a geometry closely related to that of the anion generated by removal of a proton at the 9-position. The anion is undoubtedly planar with the remaining g-hydrogen in the equatorial plane. It is thus suggested that the excited neutral molecule will have one of the 9-hydrogens in-plane (equatorial), the other out-of-plane (axial) and loosely bound.
B. Berlman, Handbookof fluorescence spectra of aromatic molecules (Academic Press, Sew York. 1965). [Z] M. Nakamizo and Y. Kanda, Spectrocbim. Acta 19 (1963) 1235. [3) D. W. Ellis and B.S. Solomon, J. Chem. Phys. 46 (1967) 3497. [4] 0. Dann and P. Nickel, Ann. Chem. 667 (1963) 101. (51I. H. Munro, ‘I’. D. S. Hamilton, J. P. Ray and G. F. Moore, Phys. Letters 20 (1966) 356. [SJH. L. Bradlow and C.A.VanderWerf. J. Am. Chem. SOC.69 (1947j 1254. [‘i] S.W. Chaikin and W.G. Brown, J. Am. Chem. Sot. 71 (1949) 122. [8] H. F. Miller and G. B.Bacbman, J. Am. Chem. Sot. 57 (1935) 2448. [S] I. B. Berlman, private communication. [lo] E. Vander Donckt, J. NasieIsM ad P.Thiry, Cbem. Common. (1969) 1249. [l]
I.
11s