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Room temperature phosphorescence of aromatics in fluid dimethylmercury
Volume 20, number 1
,
CHEMICAL PHYSICS LETTERS
1 hlny 1973
ROOM TEMPERATURE PHOSPHORESCENCE OF AROMATICS IN FLUID DE%lETHYLMERCURY E. VANDER
DONCKT,
M. MATACNE
and M. SAPIR
Collectif de Chirnie Organiqur!Physique, UtziversifhLibre de Brmelles, B-1050 Srmsels, Belgium Received 10 November 1972 Revised manuscript received 19 January
The use of a spin-forbidden
transition
1973
enhancer as dimethylmercury allows one to measure phoqhorescencc As an example, the T1 + So radiative transition ofoct~hcficcne. and 3,4-benzophenamhrcne have been studied.
spectra in a fluid medium at room temperature. benzanthracene,
1,2,5,6Cibenzanthracene
1. introduction Since the pioneer w-ork of Parker on phenanthrene [l] , the phosphorescence of naphthalene and pyrene has been studied quantitatively in liquid ethanol [2] and in the vnpour phase [3] _The experiments were performed on very sensitive spectrophosphofluorimeters making use of cooled selected photomultipliers and lock-in amplification [4] since, even in very pure solvents (impurity level including oxygen below 10mg M), the quantum yields of phosphorescence (&) are of the order of 10m6.
2. Results and discussion During our work on the fluorescence quenching of aromatic hydrocarbons by organometallic molecules, it was found that the emission spectrum of octahelicene in ethanol solution displays, in the presence of small amounts of dimethylmercury (DMM), a new emission band centred at 575 nm. Since singlet excited aromatics (IA’) form complexes with DMM (lA.DMM)* [5], the new band was tentatively assigned to an octahelicene-DMM exciplex emission. A temperature effect was performed on the rate constant of fluorescence quenching (kQ) of free octahelicene by DMM and on the ratio of the flucyescence in-
I,2-
tensities of the free aromatic (F) and of the hypothetical complexed species (F’). kq was estimated by npplication of the Stern-Volmer relationship taking 7s, = 10 nsec [O] . According to the following kinetic scheme which has proved to be applicable to other aromstic-.DMM systems [S ] : k, % ‘A* + DMM += ‘(A.DMM)+ --f 3AL f DMhi % J\ 3A*
“;
,% ‘A++
‘(A&MM)
there is a high temperature region where the relaxation time of the equilibrium between free and complexed species is short compared to their fluorescent state lifetimes. At those temperatures, one has [7, S] : kQ = (k2+k;)
exp (-AGg/RT)
(1)
and F’/ [F(DMM)]
= [k; exp (-AG&/Rr)]
lkE I
(2)
where AG& is the free’enthalpy corresponding to I(,. Providing the .exciplex lifetime is temperature independent, expressions (I) and (2) shouId yieId the same Arrhenius parameters. From fig. 1 it appears that the pIot of IogkQ versus T-l gives the usual type tif curve exhibiting highand Iqw-temperature .behaviours. From the high31
P
r s
935
.l
-s %
a 03
0
05
9.10
0
.9.05
05
9.00
-1 :
3.1
3.3.
3.5
1 May 1973
CHEMICAL PHYSICS LETTERS
Volume 20, number 1
3.7
3.8
4.1
4.3
4.5
4.7
6.95 3 -3
+X10
Fig. 1. Plot of IogkQ versus T’ according to eq. (1) (-x-x-x-) and plot of logF’/F(DM?4) versus T-’ according to eq. (2) (-o-o-*-). Solvent: 95% ethanol + 0.25 M DMM. temperature data the afi& value is estimated to be 2.6 kcal mole-l. On the other hand, if the plot of logF’/F(DMM) versus T-l is considered as reflecting the high-temperature region, the slope of the Arrhenius plot between 270 and 325°K would correof 10.6 kcal mole-l. This is a much spond to a m& larger value than those already measured for similar systems: AJl& (1,2-benzanthracene-DMM in ethanol) = 1.8 kcal mole-l and AH0 (pyrene-DMM in ethanol) = 1.5 kcal mole- pZ.5,9] . Since these results made our first assignment of the 575 nm transition doubtful, it was attempted to attribute this longwavelength emission to an octahelicene T, + So transition enhanced by the external heavy atom effect. Phosphorescence of octahelicene was thus measured in ethanol at 77’K. It was found that the iow-temperature and room-temperature emission in &id ethanolDM?V (0.25 M) match perfectly. In order to check whether this unusu? observation can be extended to other aromatics, the emission spectra of 3,4-benzophenanthrene, 1,Zbenzanthracene -. and 1,2,5,6-dibenzanthracene were recorded in DMM solution between 380 and 650 nm. This was done on fluorimeter. As light source, we use a : aconventional Hanoviz SuPerpressure 100 W mercury arc in conjunc.‘: ,tion with a dc.Hanovia stabilized power supply. The
excitation wavelength (366 nm) is selected by a Hilger and Watts D 292 grating monochromator. The lights emitted at 90” is focused on and analysed by the same type of monochromator. The monitoring photomultiplier is an RCA lP28. The signal is amplified by a 610 C Keithley electrometer and the spectra are recorded on a HR-96 Advance Electronics .X-Y recorder. The temperature of the samples is controlled by using a Pyrex dewar in which the 1 X 1 X 5 silica cells can be positioned reproducibly. The dewar is filled with ethanol or water. Circulated methanoi or water controlled the temperature between -35’C and +SO”C. The solutions are deoxygenated by the freezing, pumping, thawing technique until the air pressure above the sample remains below 10m4 torr at low temperature. A few selected emission spectra are shown in fig. 2. The fluorescence intensity is strongly quenched in DMM as solvent and in some cases the shape of the fluorescence band is modified by the superposition with an exciplex emission. The phosphorescence of the four aromatic molecules considered is observed at room or even higher temperature. Keeping in mind the very weak sensitivity of the 1P28 photomultiplier above 550 nm, the quantum yield of phosphorescence is often much larger than the quantum yield of fluorescence. The examination of phosphorescence in fluid solution in DMM permits now to study the external effect of heavy atoms on radiative and radiationless trensitions in better conditions than in-glassy solvents. Indeed, in fluid samples, the ambiguity due to inhomogeneity is avoided. The quantum yields of phosphorescence have been evaluated for two systems by comparison of the fluorescence and phosphorescence area after correction for the photomultiplier response as a function of the wavelength. The quantum yields of fluorescence in the presence of DMM were measured by comparing the area of emission of the aromatic molecules in the presence of the quencher with those recorded in ethanol as solvent. One finds: %25”c octahelicene
(ethanol+0.25
@$?’ 3,4-benzophenanthrene
M DMM) x 5 X 10m4,
(neat DMM) =Z lob3
.
The Qp values in DMM at room temperature appear to be about 100 time,s larger than that measured in ethanol as solvent [2]. Furthermore, the reduction of
CHEMICAL PHYSICS LEI-I-ERS
Volume 20, number 1
1 May 1973
0.8 Ie 06
Fig. 2. Relative emission spectra in fluid solvents without correction for the sensitivity of the detection system as a function of the wavelength. m ethanol, normalized at I, (4.55 run) = I ;@ in A. Octalzelicene. Re!arive intensities (I,) cf the emission spectra at -6S”C.a’ at +25”C normalized at Ie (455 nm) = 1; sohent: ethanol f ethanol + 0.25 M D%ihi;@- in ethanol + 0.68 hl DbIM;@spectrum 0.68 hl DMM. B. 1,2,5,6-diberl,-antlIrace,le.OIn benzene at 25”C;I, (399 nm) = 13 in Dhlhf, normalized at fe (399 nm) = 0.1. temperature: -2O”Cf.. in Dhlhl, normalized at I, (399 nm) = 0.1, temperature: +15”C;.‘3 in DMM, normalized et fe (394 nm) = 0.1, temperature: +25”C. C. 1,2-benlanrhrocelle.O In benzene at 25’C, fe (395 nm) = 0.7@ in DhfjM, nonnalired at fe (395 nm) = 0.7, temperscure: in Dhlhl, normalized at fe (395 nm) = 0.7. tempcra-2O”C@ in DMM, normalized at I, (39.5 run) = 0.7, temperature: +25’Ci,g ture: +73”C. D. 3,4-br)zZoPheno,lrhre,re.O1~ ethanol at 25”C, I, (395 nm) = lp$iin Dhlhi, nonalized at fe (395 nm) = 0.5, temperature: -34”C;ain DXIM, normalized at 1,: (395 nm) = 0.5, temperature: +24”C.
the triplet-state
lifetime in DMM is such that the Tl is observable under common experimental conditions. In particular, the sensitivity to impurities is dramatically reduced. The attempts to detect the triplet-triplet absorption of the aromatics in DMM solution by conventional flash photolysis (time resolution = 5 X 10e5 set) were unsuccessful. Although the quantum yield of triplet-state formation of aromatics reaches a value of unity in this medium [lo] and the T-T absorption is observed in usual solvents (benzene, ethanol).
Acknowledgement
+ So emission
The authors are indebted to Professor R.H. Martin for his interest and Professor 1. NasieIski for useful conversations. We thank Mr. R. VandeIoise for carrying out the tuning of the tluorometer and the I.R.S.I.A. for maintenance grants (MM. and h4.S.). References [ I] CA. Parker, Chem. Britain 2 (1966) 160; Photoluminescence in solution (Elsevier, Amsterdam. 1968).
83
.
~ol~e2O,m_&ber
,_ ‘.
1
CHEMICAL PHYSICS LETTERS
i May
1973
.’ [2] 3. Langelaar, R.P.H. Rettschnick, A.M.F. Lambooy and GJ. Hoytink, Chem. Phys. Letters l(1968) 609; J. Langelaar, R.P.H. Rettschnlck and G.J. Hoytink, J. Chem. Phys. 54 (1971) 1. [ 31 W.H. van Lceuwen. J. Langelaar and J.D.W. van Voorst, Chem. Phys Letters 13 (1972) 622. [4] J. Langelaar, G.A. de Vries and D. Bebchar, J. Sci Instr. 2 (1969) 149. [S j E. Vander Donckt, D. Li&aer and M. Matagne, J. Chem. Sot Faraday Trans, submitted for publication.
[6] E. Vander Lk&kt, J. Nasielski, J.R. Greenleaf and J.B. Birks, Chem. Phys. Letters 2 (1968) 409. [7] Th. Forster and K. Kasper, 2. Elektrochem. 59 (1955) 976. [8] J.B. Birka, Photophysics of aromatic molecules (WileyInterscience, New York, 1970). [V] P. Toussaint and E. Vander Donckt, unpublished data [lo] E. Vander Donckt and D. Lie’taer, J. Chem. Sot. Faraday Trans I 68 (1972) 112.
,
CHEMICAL PHYSICS LETTERS
1 hlny 1973
ROOM TEMPERATURE PHOSPHORESCENCE OF AROMATICS IN FLUID DE%lETHYLMERCURY E. VANDER
DONCKT,
M. MATACNE
and M. SAPIR
Collectif de Chirnie Organiqur!Physique, UtziversifhLibre de Brmelles, B-1050 Srmsels, Belgium Received 10 November 1972 Revised manuscript received 19 January
The use of a spin-forbidden
transition
1973
enhancer as dimethylmercury allows one to measure phoqhorescencc As an example, the T1 + So radiative transition ofoct~hcficcne. and 3,4-benzophenamhrcne have been studied.
spectra in a fluid medium at room temperature. benzanthracene,
1,2,5,6Cibenzanthracene
1. introduction Since the pioneer w-ork of Parker on phenanthrene [l] , the phosphorescence of naphthalene and pyrene has been studied quantitatively in liquid ethanol [2] and in the vnpour phase [3] _The experiments were performed on very sensitive spectrophosphofluorimeters making use of cooled selected photomultipliers and lock-in amplification [4] since, even in very pure solvents (impurity level including oxygen below 10mg M), the quantum yields of phosphorescence (&) are of the order of 10m6.
2. Results and discussion During our work on the fluorescence quenching of aromatic hydrocarbons by organometallic molecules, it was found that the emission spectrum of octahelicene in ethanol solution displays, in the presence of small amounts of dimethylmercury (DMM), a new emission band centred at 575 nm. Since singlet excited aromatics (IA’) form complexes with DMM (lA.DMM)* [5], the new band was tentatively assigned to an octahelicene-DMM exciplex emission. A temperature effect was performed on the rate constant of fluorescence quenching (kQ) of free octahelicene by DMM and on the ratio of the flucyescence in-
I,2-
tensities of the free aromatic (F) and of the hypothetical complexed species (F’). kq was estimated by npplication of the Stern-Volmer relationship taking 7s, = 10 nsec [O] . According to the following kinetic scheme which has proved to be applicable to other aromstic-.DMM systems [S ] : k, % ‘A* + DMM += ‘(A.DMM)+ --f 3AL f DMhi % J\ 3A*
“;
,% ‘A++
‘(A&MM)
there is a high temperature region where the relaxation time of the equilibrium between free and complexed species is short compared to their fluorescent state lifetimes. At those temperatures, one has [7, S] : kQ = (k2+k;)
exp (-AGg/RT)
(1)
and F’/ [F(DMM)]
= [k; exp (-AG&/Rr)]
lkE I
(2)
where AG& is the free’enthalpy corresponding to I(,. Providing the .exciplex lifetime is temperature independent, expressions (I) and (2) shouId yieId the same Arrhenius parameters. From fig. 1 it appears that the pIot of IogkQ versus T-l gives the usual type tif curve exhibiting highand Iqw-temperature .behaviours. From the high31
P
r s
935
.l
-s %
a 03
0
05
9.10
0
.9.05
05
9.00
-1 :
3.1
3.3.
3.5
1 May 1973
CHEMICAL PHYSICS LETTERS
Volume 20, number 1
3.7
3.8
4.1
4.3
4.5
4.7
6.95 3 -3
+X10
Fig. 1. Plot of IogkQ versus T’ according to eq. (1) (-x-x-x-) and plot of logF’/F(DM?4) versus T-’ according to eq. (2) (-o-o-*-). Solvent: 95% ethanol + 0.25 M DMM. temperature data the afi& value is estimated to be 2.6 kcal mole-l. On the other hand, if the plot of logF’/F(DMM) versus T-l is considered as reflecting the high-temperature region, the slope of the Arrhenius plot between 270 and 325°K would correof 10.6 kcal mole-l. This is a much spond to a m& larger value than those already measured for similar systems: AJl& (1,2-benzanthracene-DMM in ethanol) = 1.8 kcal mole-l and AH0 (pyrene-DMM in ethanol) = 1.5 kcal mole- pZ.5,9] . Since these results made our first assignment of the 575 nm transition doubtful, it was attempted to attribute this longwavelength emission to an octahelicene T, + So transition enhanced by the external heavy atom effect. Phosphorescence of octahelicene was thus measured in ethanol at 77’K. It was found that the iow-temperature and room-temperature emission in &id ethanolDM?V (0.25 M) match perfectly. In order to check whether this unusu? observation can be extended to other aromatics, the emission spectra of 3,4-benzophenanthrene, 1,Zbenzanthracene -. and 1,2,5,6-dibenzanthracene were recorded in DMM solution between 380 and 650 nm. This was done on fluorimeter. As light source, we use a : aconventional Hanoviz SuPerpressure 100 W mercury arc in conjunc.‘: ,tion with a dc.Hanovia stabilized power supply. The
excitation wavelength (366 nm) is selected by a Hilger and Watts D 292 grating monochromator. The lights emitted at 90” is focused on and analysed by the same type of monochromator. The monitoring photomultiplier is an RCA lP28. The signal is amplified by a 610 C Keithley electrometer and the spectra are recorded on a HR-96 Advance Electronics .X-Y recorder. The temperature of the samples is controlled by using a Pyrex dewar in which the 1 X 1 X 5 silica cells can be positioned reproducibly. The dewar is filled with ethanol or water. Circulated methanoi or water controlled the temperature between -35’C and +SO”C. The solutions are deoxygenated by the freezing, pumping, thawing technique until the air pressure above the sample remains below 10m4 torr at low temperature. A few selected emission spectra are shown in fig. 2. The fluorescence intensity is strongly quenched in DMM as solvent and in some cases the shape of the fluorescence band is modified by the superposition with an exciplex emission. The phosphorescence of the four aromatic molecules considered is observed at room or even higher temperature. Keeping in mind the very weak sensitivity of the 1P28 photomultiplier above 550 nm, the quantum yield of phosphorescence is often much larger than the quantum yield of fluorescence. The examination of phosphorescence in fluid solution in DMM permits now to study the external effect of heavy atoms on radiative and radiationless trensitions in better conditions than in-glassy solvents. Indeed, in fluid samples, the ambiguity due to inhomogeneity is avoided. The quantum yields of phosphorescence have been evaluated for two systems by comparison of the fluorescence and phosphorescence area after correction for the photomultiplier response as a function of the wavelength. The quantum yields of fluorescence in the presence of DMM were measured by comparing the area of emission of the aromatic molecules in the presence of the quencher with those recorded in ethanol as solvent. One finds: %25”c octahelicene
(ethanol+0.25
@$?’ 3,4-benzophenanthrene
M DMM) x 5 X 10m4,
(neat DMM) =Z lob3
.
The Qp values in DMM at room temperature appear to be about 100 time,s larger than that measured in ethanol as solvent [2]. Furthermore, the reduction of
CHEMICAL PHYSICS LEI-I-ERS
Volume 20, number 1
1 May 1973
0.8 Ie 06
Fig. 2. Relative emission spectra in fluid solvents without correction for the sensitivity of the detection system as a function of the wavelength. m ethanol, normalized at I, (4.55 run) = I ;@ in A. Octalzelicene. Re!arive intensities (I,) cf the emission spectra at -6S”C.a’ at +25”C normalized at Ie (455 nm) = 1; sohent: ethanol f ethanol + 0.25 M D%ihi;@- in ethanol + 0.68 hl DbIM;@spectrum 0.68 hl DMM. B. 1,2,5,6-diberl,-antlIrace,le.OIn benzene at 25”C;I, (399 nm) = 13 in Dhlhf, normalized at fe (399 nm) = 0.1. temperature: -2O”Cf.. in Dhlhl, normalized at I, (399 nm) = 0.1, temperature: +15”C;.‘3 in DMM, normalized et fe (394 nm) = 0.1, temperature: +25”C. C. 1,2-benlanrhrocelle.O In benzene at 25’C, fe (395 nm) = 0.7@ in DhfjM, nonnalired at fe (395 nm) = 0.7, temperscure: in Dhlhl, normalized at fe (395 nm) = 0.7. tempcra-2O”C@ in DMM, normalized at I, (39.5 run) = 0.7, temperature: +25’Ci,g ture: +73”C. D. 3,4-br)zZoPheno,lrhre,re.O1~ ethanol at 25”C, I, (395 nm) = lp$iin Dhlhi, nonalized at fe (395 nm) = 0.5, temperature: -34”C;ain DXIM, normalized at 1,: (395 nm) = 0.5, temperature: +24”C.
the triplet-state
lifetime in DMM is such that the Tl is observable under common experimental conditions. In particular, the sensitivity to impurities is dramatically reduced. The attempts to detect the triplet-triplet absorption of the aromatics in DMM solution by conventional flash photolysis (time resolution = 5 X 10e5 set) were unsuccessful. Although the quantum yield of triplet-state formation of aromatics reaches a value of unity in this medium [lo] and the T-T absorption is observed in usual solvents (benzene, ethanol).
Acknowledgement
+ So emission
The authors are indebted to Professor R.H. Martin for his interest and Professor 1. NasieIski for useful conversations. We thank Mr. R. VandeIoise for carrying out the tuning of the tluorometer and the I.R.S.I.A. for maintenance grants (MM. and h4.S.). References [ I] CA. Parker, Chem. Britain 2 (1966) 160; Photoluminescence in solution (Elsevier, Amsterdam. 1968).
83
.
~ol~e2O,m_&ber
,_ ‘.
1
CHEMICAL PHYSICS LETTERS
i May
1973
.’ [2] 3. Langelaar, R.P.H. Rettschnick, A.M.F. Lambooy and GJ. Hoytink, Chem. Phys. Letters l(1968) 609; J. Langelaar, R.P.H. Rettschnlck and G.J. Hoytink, J. Chem. Phys. 54 (1971) 1. [ 31 W.H. van Lceuwen. J. Langelaar and J.D.W. van Voorst, Chem. Phys Letters 13 (1972) 622. [4] J. Langelaar, G.A. de Vries and D. Bebchar, J. Sci Instr. 2 (1969) 149. [S j E. Vander Donckt, D. Li&aer and M. Matagne, J. Chem. Sot Faraday Trans, submitted for publication.
[6] E. Vander Lk&kt, J. Nasielski, J.R. Greenleaf and J.B. Birks, Chem. Phys. Letters 2 (1968) 409. [7] Th. Forster and K. Kasper, 2. Elektrochem. 59 (1955) 976. [8] J.B. Birka, Photophysics of aromatic molecules (WileyInterscience, New York, 1970). [V] P. Toussaint and E. Vander Donckt, unpublished data [lo] E. Vander Donckt and D. Lie’taer, J. Chem. Sot. Faraday Trans I 68 (1972) 112.