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Efficient quenching of luminescence by so-called “inert” fluorocarbon solvents
Volume 55, number I
CHEMICAL
PHYSICS
LETTERS
I April 1978
EFFICIENT QUENCHING OF LUMINESCENCE BY SO-CALLED “WERT” FLUOROCARBON SOLVENTS PC. ALFORD,
C-G. CLIRETON, R-A. L_AMPERT and D. PHILLIPS
Deparrnwnr of Chwzisrr_v, The Umkersit_v. Southampton. Received
19 December
SO9 SNH, UK
1977
The fluorescence of triethylamine has been shown to be quenched efficiently both in vapour phase and cyciohexane solution by the “inert” molecule perfluoromethyl cyclohexane. Bimolecular rate constants are (2-5 i 0.2) X IO” Qmol-‘s-t in the vapour phase and (1.3 z 0.2) X lOto Qrnol-‘~-~ m - solution, close to the gas kinetic and diffusion controlled rates respectively. Charge-transfer from amine to fluorocarbon is a likely quenching mechanism, and thus “inert” fluorocarbon solvents must be used with care in systems of low ionization potential such as amines.
1. Introduction Perfluorinated materials are often employed as solvents in studies on the photoluminescence of a variety of organic molecules when an inert, non-polar solvent without abstractable hydrogen atoms is required [I]. Fully fluorinated solvents have been very successful in this regard but we wish to report that there is at least one class of luminescent materials namely aliphatic amines, in which the luminescence is very strongly quenched by perfluorocarbons. Indeed, this study was prompted by the observation that in neat peffluoromethylcyclchexane, amines are totally non-fluorescent. Simple tertiary amines such as trimethyl and triethylamine are highly fluorescent [2,3] when excited in the 220-250 nm region, the absorption corresponding to Rydberg 12-3s transition [4]. Thils the quantum yield of fluorescence from these amines are 0.97 and 0.98 respectively for excitation at 248 nm [ 1,3] and they provide good models for studies in luminescence quenching. We report here the effect of perfluorocarbons upon the fluorescence of triethylamine excited at 240 nm.
prising 150 W Xe arc exciting lamp, Bausch and Lomb high intensity monochromator for wavelength selection, T-shaped quartz fluorescence cell, RCA 935 photodiode for monitoring transmitted light, and RCA 1P28 photomultiplier for emission measurements_ A similar set-up was used for solution phase measurements, but in a 1 cm quartz cuvette. Samples were prepared on a vacuum system using mercury diffusion pumps. Fluorescence decay times were measured using the time-correlated single-photon counting method on an apparatus described adequately elsewhere [S].
3. Materials Triethylamine (BDH Ltd.) was purified by distillation over potassium hydroxide. Perfluorocyclohexane (Flutec) was passed through an alumina column, and cyclohexane (BDH Spectroscopic) through a silica column at least twice for purification. All materials were thoroughly outgassed by freeze-pump-thaw cycles before use.
4. Results and discussion 2. Experimental Relative fluorescence quantum yields were measured in the vapour phase on an optical system com100
In the vapour phase the fluorescence of triethylamine is quenched efficientiy by perfluoromethylcyclohexane as shown in fig_ 1, in which reciprocal fluo-
Volume 55, number 1
z
Prsssure
CHEMICAL
PHYSICS
PFMCH 1 torr
Fig. 1. Reciprocal fluorescence decay time and relative quantum yield of triethylamine vapour as function of pressure of added perfluoromethylcyclohesane. Decay times, icft hand scale, relative yields, rigt hand. Excitation wavelength o, 240 run, - 230 nm. Pressure of triethylamine 1 torr for decay measurements, 2 torr for yield measurements.
rescence decay tires are plotted as a function of pressure of additive. It should be noted that the decay time for 1 torr of the particular sample of triethylamine in the absence of perfluoromethyl cyclohexane measured here is 44 ns, somewhat shorter than that reported earlier [3], possibly indicating some residual impurity. The slope of the line in fig. 1 gives a secondorder rate constant for quenching of (2-7 + 0.2) X 10” Qrnol-‘~-~, close to the gas kinetic hard sphere collision rate. This result is confirmed by observation of diminution of fluorescence intensity as a function of concentration of perfluoromethylcyclohexane also shown
in fig_ 1_ The Stem-Volmer
slopes kq7
from
these plots are.1 -10 X IO4 P mol-I, for 240 nm excitation and 1.47 X IO4 Qmol-l for 230 nm excitation which when combined with the decay times measured here on separate samples of amine of differing purity for 2 torr amine gives kQ as(2.3 f 0.3) X 10” Rmol-’ s-l for 240 run excitation, (2.6 i 0.2) X 10 l1 Qmol-1 s-l for 230 nm excitation respectively. In solution, a similar situation exists. The fluorescence intensity from 6 X 10d4 M triethylamine in cyclohexane solution obeys a good Stem-Volmer rela-
LElTERS
1 April 1978
tionship with addition of perfluoromethylcyclohexane, the Stem-Volmer slope being 2.24 X lo2 P molW1_Combined with a measured fluorescence decay time of 17.2 ns for this concentration of triethylamine in cyclohexane this gives a second-order rate constant for quenching of (1.3 20.2) X lOLo Qmol-lsM1, of the order of the diffusion controlled rate for cyclohexane [6]Further studies indicate that triethjrlamine fluorescence is also very efficiently quenched by the perfluorocarbons octafluorocyclobutane, perfluoro-but-2ene and also by aromatic molecules containing trifluoromethyl groups. Since all of these species are characterized by having high electron affinities, it is reasonable in view of the low ionization potential of the amine to regard the quenching as due to charge transfer or even electron transfer from excited amine to fluorocarbon. There is no evidence of ground state association in absorption spectra, so this must be a purely excited state phenomenon_ Evidently perfluoro solvents are not inert with respect to excited state species of low ionization potential, and care must be exercised in their use with such systems.
Acknowledgement We are grateful to the Science Research Council for equipment grants and for a Research Studentship to C.G.C.
References [l] J.P. Blanchi and A.R. Watkins, Chem. Commun. [21 (31 [4] IS] 161
(1974) 265. A.M. Halpern, Mol. Photochem. 5 (1973) 517. A.M. Halpern and T. Cartman, J. Am. Chem. SOC.96. (1974) 1393. A.M. Halpem, J.L. Roebber and K. Weiss, J. Chem. Phys. 49 (1968) 1348. P.A. Hacketr and D. Phillips, J. Pbys. Chem. 78 (1974) 671. S.L. hkrov, Handbook of photochemistry (Dekker, New York, 1973) p_ 55.
101
CHEMICAL
PHYSICS
LETTERS
I April 1978
EFFICIENT QUENCHING OF LUMINESCENCE BY SO-CALLED “WERT” FLUOROCARBON SOLVENTS PC. ALFORD,
C-G. CLIRETON, R-A. L_AMPERT and D. PHILLIPS
Deparrnwnr of Chwzisrr_v, The Umkersit_v. Southampton. Received
19 December
SO9 SNH, UK
1977
The fluorescence of triethylamine has been shown to be quenched efficiently both in vapour phase and cyciohexane solution by the “inert” molecule perfluoromethyl cyclohexane. Bimolecular rate constants are (2-5 i 0.2) X IO” Qmol-‘s-t in the vapour phase and (1.3 z 0.2) X lOto Qrnol-‘~-~ m - solution, close to the gas kinetic and diffusion controlled rates respectively. Charge-transfer from amine to fluorocarbon is a likely quenching mechanism, and thus “inert” fluorocarbon solvents must be used with care in systems of low ionization potential such as amines.
1. Introduction Perfluorinated materials are often employed as solvents in studies on the photoluminescence of a variety of organic molecules when an inert, non-polar solvent without abstractable hydrogen atoms is required [I]. Fully fluorinated solvents have been very successful in this regard but we wish to report that there is at least one class of luminescent materials namely aliphatic amines, in which the luminescence is very strongly quenched by perfluorocarbons. Indeed, this study was prompted by the observation that in neat peffluoromethylcyclchexane, amines are totally non-fluorescent. Simple tertiary amines such as trimethyl and triethylamine are highly fluorescent [2,3] when excited in the 220-250 nm region, the absorption corresponding to Rydberg 12-3s transition [4]. Thils the quantum yield of fluorescence from these amines are 0.97 and 0.98 respectively for excitation at 248 nm [ 1,3] and they provide good models for studies in luminescence quenching. We report here the effect of perfluorocarbons upon the fluorescence of triethylamine excited at 240 nm.
prising 150 W Xe arc exciting lamp, Bausch and Lomb high intensity monochromator for wavelength selection, T-shaped quartz fluorescence cell, RCA 935 photodiode for monitoring transmitted light, and RCA 1P28 photomultiplier for emission measurements_ A similar set-up was used for solution phase measurements, but in a 1 cm quartz cuvette. Samples were prepared on a vacuum system using mercury diffusion pumps. Fluorescence decay times were measured using the time-correlated single-photon counting method on an apparatus described adequately elsewhere [S].
3. Materials Triethylamine (BDH Ltd.) was purified by distillation over potassium hydroxide. Perfluorocyclohexane (Flutec) was passed through an alumina column, and cyclohexane (BDH Spectroscopic) through a silica column at least twice for purification. All materials were thoroughly outgassed by freeze-pump-thaw cycles before use.
4. Results and discussion 2. Experimental Relative fluorescence quantum yields were measured in the vapour phase on an optical system com100
In the vapour phase the fluorescence of triethylamine is quenched efficientiy by perfluoromethylcyclohexane as shown in fig_ 1, in which reciprocal fluo-
Volume 55, number 1
z
Prsssure
CHEMICAL
PHYSICS
PFMCH 1 torr
Fig. 1. Reciprocal fluorescence decay time and relative quantum yield of triethylamine vapour as function of pressure of added perfluoromethylcyclohesane. Decay times, icft hand scale, relative yields, rigt hand. Excitation wavelength o, 240 run, - 230 nm. Pressure of triethylamine 1 torr for decay measurements, 2 torr for yield measurements.
rescence decay tires are plotted as a function of pressure of additive. It should be noted that the decay time for 1 torr of the particular sample of triethylamine in the absence of perfluoromethyl cyclohexane measured here is 44 ns, somewhat shorter than that reported earlier [3], possibly indicating some residual impurity. The slope of the line in fig. 1 gives a secondorder rate constant for quenching of (2-7 + 0.2) X 10” Qrnol-‘~-~, close to the gas kinetic hard sphere collision rate. This result is confirmed by observation of diminution of fluorescence intensity as a function of concentration of perfluoromethylcyclohexane also shown
in fig_ 1_ The Stem-Volmer
slopes kq7
from
these plots are.1 -10 X IO4 P mol-I, for 240 nm excitation and 1.47 X IO4 Qmol-l for 230 nm excitation which when combined with the decay times measured here on separate samples of amine of differing purity for 2 torr amine gives kQ as(2.3 f 0.3) X 10” Rmol-’ s-l for 240 run excitation, (2.6 i 0.2) X 10 l1 Qmol-1 s-l for 230 nm excitation respectively. In solution, a similar situation exists. The fluorescence intensity from 6 X 10d4 M triethylamine in cyclohexane solution obeys a good Stem-Volmer rela-
LElTERS
1 April 1978
tionship with addition of perfluoromethylcyclohexane, the Stem-Volmer slope being 2.24 X lo2 P molW1_Combined with a measured fluorescence decay time of 17.2 ns for this concentration of triethylamine in cyclohexane this gives a second-order rate constant for quenching of (1.3 20.2) X lOLo Qmol-lsM1, of the order of the diffusion controlled rate for cyclohexane [6]Further studies indicate that triethjrlamine fluorescence is also very efficiently quenched by the perfluorocarbons octafluorocyclobutane, perfluoro-but-2ene and also by aromatic molecules containing trifluoromethyl groups. Since all of these species are characterized by having high electron affinities, it is reasonable in view of the low ionization potential of the amine to regard the quenching as due to charge transfer or even electron transfer from excited amine to fluorocarbon. There is no evidence of ground state association in absorption spectra, so this must be a purely excited state phenomenon_ Evidently perfluoro solvents are not inert with respect to excited state species of low ionization potential, and care must be exercised in their use with such systems.
Acknowledgement We are grateful to the Science Research Council for equipment grants and for a Research Studentship to C.G.C.
References [l] J.P. Blanchi and A.R. Watkins, Chem. Commun. [21 (31 [4] IS] 161
(1974) 265. A.M. Halpern, Mol. Photochem. 5 (1973) 517. A.M. Halpern and T. Cartman, J. Am. Chem. SOC.96. (1974) 1393. A.M. Halpem, J.L. Roebber and K. Weiss, J. Chem. Phys. 49 (1968) 1348. P.A. Hacketr and D. Phillips, J. Pbys. Chem. 78 (1974) 671. S.L. hkrov, Handbook of photochemistry (Dekker, New York, 1973) p_ 55.
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