- Email: [email protected]
Effect of pressure on viscosity-dependent dye fluorescence
Volume46, number 3
CHEMIC!AL PHYSICSLETTERS
15 March 1977
EFFECT OF PRESSURE ON VISCOSITY-DEPENDENTDYE FLUORESCENCE
Rcccivcd 6 December 1976
The ~uore~e~ce of the dyes crystal violet, auramint+ and cask-?’ has been studied 1s a Function of prewrrc in glycerol and ~ol~nyIa~~ho1 at room temperature. ‘The strong pressure sensitivity observed for the substttuted meWne dyes is reWed to the viscosity change of the ffuid medium.
It is well known that substituted di- and triphenyf methane dyes show exceptionally low quantum yields in fluid solutions, yet become strong emitters in rigid media. Two examples are auramine- studied by Oster and Nishijima [I] and crystal violet investigated by Forster and Hoffmann [2 1. The fluidity of the solution was modi~ed by changes in solvent composition [I f or by variations in temperature [2;, and indicated by the measurement of the macroscopic viscosity. Since temperature and specific solvent interactions can influence the radiative and radiationless processes of electronic&y excited molecules, the pressure parameter has been used here as an attemate to modify the fluidity. The present work compares the effect of pressure to 5 100 bar on the fluorescence of crystal violet (CV) and auramine- (AO) in glycerol at 22’C with that of eosin-Y (EY) under the same conditions (fig. I). Solutions were prepared to be 1 X fW4 M in glycerol (Matheson, Coleman and BeII, spectroquality grade). Previous studies [1,3] suggest that concentration quenching should be insignificant at this concentration. Ihe pressure-induced change in ffuorescence was also measured in ~o~y~ny!aI~oho~(WA) to permit comparison of the results in glycerol with a rigid medium. The viscosity of glycerol, known to be hygroscopic was determined to be 8.99 poise at 22.O”C and 1 atm in a standard Ostwald viscosimeter. This value is in excellent agreement with the extr~~o~at~ value from aeely’s [4] me~urement of 8.67 poise at 22S°C for
Fig. I. TIIC structure of and eosin-Y EY)
crystal
violet
(CV),auramind (AO),
a glycerol sample of the same specific gravity (I -2582 ~~m3). The pressure dependence of %e ~s~os~ty of glycerol has been determined by Bridgman 153 at 30°C and found to be in reasonable agreement (within 10%) with the empirical relationship put forward by Zolotykb 161 (I)
Volume 46, number 3
CHEMICALPHYSICSLETTERS
1sMarch1977
Table 1 Fluorescence data for the three dyes Excitation wavelength
Emission peak
Approximate red-shift
Relative intemity (fir,)
(nm)
(nm)
in glycerol
glycerol 5.1 kbar
WA
IO 6
C.0 1.3
-__crystal violet auramineeosin-Y
(cm-‘/kbar)
---_
365
630 495
320
544
530
where q. is the viscosity value at 1 bar and the constant u = 0.000537 bar-E at 22°C extrapolated from his values given at 23 and 30°C [6]. The results are shown in table 1 and fig. 2. The strong fluorescence enhancement under pressure is clearIy evident for CV and A0 in fluid sohrtion. The intensity data (f at pressure P and lo at 1 atm) for
-2-S -40 -20
.
I
.
IO
20
30
PRESSURE
.
40
.
5.0
i KBAR)
Fig. 2. Relative fluorescence intensities I& at six pressures for crystal violet (a), auramine (A), and eosin-Y (0). Also given are the pressure dependence of q/vu according to eq. (I) (.-.-) and I/&Jaccording to eq. (3) (for AO) (- --) and eq. (4) (for CV) (---).
1.4
these two dyes in fluid solution can be fit by the equation III0 = b exp(cP),
(2)
to give for P (in kbar) the constants b and c equal to 1.25 and 0.441 for CV and 1.12 and 0.355 Far AO, respectively. Ihe pressure effect is c!earEy less evident in plastic and for EY in both media. The I/f0 values for the glycerol solution and plastic arc recorded in table 1 for roughly the same volume contraction of 7% according to values reported by Bridgman [71 for glycerol and by Weir 181 for PVA. Earlier discussion of dyes with small quantum yieIds in fluid solutions has focused on steric effects [I ,2.9 1 e.g., crowding in non-planar molecules, re[ating steric quenching to enhanced radiationless transitions. In the methane dyes CV and A0 under present investigation the intramolecular motion of the phenyl groups is assumed to cause the very weak fluorescence Errfluids. Ckter and Nishijima [ 11 assumed that rotational diffusion of the phcnyl groups was responsible for the quenching and related their quantum yield p measurements for A0 to the equation @=A
I
0.8
10 kblr
+BTfq,
(3)
where A and B are constant, T the tez_nerature, and r) the solvent viscosity. Fiirstcr and EEoFfmann [21 challenge the assumption of free rotational diffusion and suggest an improved model based on osciltatory motion in a quadratic potential for the phenyl groups. An approximate equation, for the case of predominant radiationless transitions, is derived as q=
clj2f3 )
where Cis a constant. This equation was satisfactory in describing their results for several dyes beIonging to the CV class [2]. 589
Volume 46. number 3
CHEMICAL PHYSICS LETTERS
Fig. 2 demonstrates that both eqs. (3) and (4) are inadequate to describe the pressure dependence of I/lo for A0 and CV, respectively. These comparisons assume that isothermal compression only modifies the viscosity of the solution and that the constants in eqs. (3) and (4) are essentially pressure-independent. However, the experimental results could be fit to eq. (4), if C were assumed to be a pressure-dependent variable. ‘Ihe dependence of the fluorescence yield of substituted methane dyes on fluidity of the environment has been observed by modification of the solvent viscosity by temperature, chemical composition, and now pressure. These dyes may be used as fluorescent probes of the microviscosity of the medium in pressure zxperiments or in other studies, where it is necessary to produce a viscous medium by compression.
590
15 March 1977
References 11) G. Oster and Y. Nishijima, J. Am. Chcm. Sot. 78 (1956) 1581. 121 T?I. Farster and G. Hoffmann. 2. Physik. Chem. NF 75 (1971) 63. rl\ D. Ma&de and M.W. Windsor, Chcm. Phys. Letters 24 ;“I (1974) 144.
r41 M-L. Sheely, Ind. Eng. Chcm. 24 (1932) 1060. ISI P-W. Bridgman. Proc. Am. Awd. Arts Sci. 61 (1926) 57, _ 161 E.V. Zolotykh, Izmerit. Tckhn. 3 (1955) no. 2. I71 P.W. Btidgman. The physics of high prcssurc (Bell, London, 1949).
(81 C.E. Weir. J. Res. Natl. Bur. Std. 53 (1954) 245. PI L.J.E. Hofcr, R.J. Gnbcnstetter and E.O. Wug, J. Am. C&em. Sot. 72 (1950) 203.
CHEMIC!AL PHYSICSLETTERS
15 March 1977
EFFECT OF PRESSURE ON VISCOSITY-DEPENDENTDYE FLUORESCENCE
Rcccivcd 6 December 1976
The ~uore~e~ce of the dyes crystal violet, auramint+ and cask-?’ has been studied 1s a Function of prewrrc in glycerol and ~ol~nyIa~~ho1 at room temperature. ‘The strong pressure sensitivity observed for the substttuted meWne dyes is reWed to the viscosity change of the ffuid medium.
It is well known that substituted di- and triphenyf methane dyes show exceptionally low quantum yields in fluid solutions, yet become strong emitters in rigid media. Two examples are auramine- studied by Oster and Nishijima [I] and crystal violet investigated by Forster and Hoffmann [2 1. The fluidity of the solution was modi~ed by changes in solvent composition [I f or by variations in temperature [2;, and indicated by the measurement of the macroscopic viscosity. Since temperature and specific solvent interactions can influence the radiative and radiationless processes of electronic&y excited molecules, the pressure parameter has been used here as an attemate to modify the fluidity. The present work compares the effect of pressure to 5 100 bar on the fluorescence of crystal violet (CV) and auramine- (AO) in glycerol at 22’C with that of eosin-Y (EY) under the same conditions (fig. I). Solutions were prepared to be 1 X fW4 M in glycerol (Matheson, Coleman and BeII, spectroquality grade). Previous studies [1,3] suggest that concentration quenching should be insignificant at this concentration. Ihe pressure-induced change in ffuorescence was also measured in ~o~y~ny!aI~oho~(WA) to permit comparison of the results in glycerol with a rigid medium. The viscosity of glycerol, known to be hygroscopic was determined to be 8.99 poise at 22.O”C and 1 atm in a standard Ostwald viscosimeter. This value is in excellent agreement with the extr~~o~at~ value from aeely’s [4] me~urement of 8.67 poise at 22S°C for
Fig. I. TIIC structure of and eosin-Y EY)
crystal
violet
(CV),auramind (AO),
a glycerol sample of the same specific gravity (I -2582 ~~m3). The pressure dependence of %e ~s~os~ty of glycerol has been determined by Bridgman 153 at 30°C and found to be in reasonable agreement (within 10%) with the empirical relationship put forward by Zolotykb 161 (I)
Volume 46, number 3
CHEMICALPHYSICSLETTERS
1sMarch1977
Table 1 Fluorescence data for the three dyes Excitation wavelength
Emission peak
Approximate red-shift
Relative intemity (fir,)
(nm)
(nm)
in glycerol
glycerol 5.1 kbar
WA
IO 6
C.0 1.3
-__crystal violet auramineeosin-Y
(cm-‘/kbar)
---_
365
630 495
320
544
530
where q. is the viscosity value at 1 bar and the constant u = 0.000537 bar-E at 22°C extrapolated from his values given at 23 and 30°C [6]. The results are shown in table 1 and fig. 2. The strong fluorescence enhancement under pressure is clearIy evident for CV and A0 in fluid sohrtion. The intensity data (f at pressure P and lo at 1 atm) for
-2-S -40 -20
.
I
.
IO
20
30
PRESSURE
.
40
.
5.0
i KBAR)
Fig. 2. Relative fluorescence intensities I& at six pressures for crystal violet (a), auramine (A), and eosin-Y (0). Also given are the pressure dependence of q/vu according to eq. (I) (.-.-) and I/&Jaccording to eq. (3) (for AO) (- --) and eq. (4) (for CV) (---).
1.4
these two dyes in fluid solution can be fit by the equation III0 = b exp(cP),
(2)
to give for P (in kbar) the constants b and c equal to 1.25 and 0.441 for CV and 1.12 and 0.355 Far AO, respectively. Ihe pressure effect is c!earEy less evident in plastic and for EY in both media. The I/f0 values for the glycerol solution and plastic arc recorded in table 1 for roughly the same volume contraction of 7% according to values reported by Bridgman [71 for glycerol and by Weir 181 for PVA. Earlier discussion of dyes with small quantum yieIds in fluid solutions has focused on steric effects [I ,2.9 1 e.g., crowding in non-planar molecules, re[ating steric quenching to enhanced radiationless transitions. In the methane dyes CV and A0 under present investigation the intramolecular motion of the phenyl groups is assumed to cause the very weak fluorescence Errfluids. Ckter and Nishijima [ 11 assumed that rotational diffusion of the phcnyl groups was responsible for the quenching and related their quantum yield p measurements for A0 to the equation @=A
I
0.8
10 kblr
+BTfq,
(3)
where A and B are constant, T the tez_nerature, and r) the solvent viscosity. Fiirstcr and EEoFfmann [21 challenge the assumption of free rotational diffusion and suggest an improved model based on osciltatory motion in a quadratic potential for the phenyl groups. An approximate equation, for the case of predominant radiationless transitions, is derived as q=
clj2f3 )
where Cis a constant. This equation was satisfactory in describing their results for several dyes beIonging to the CV class [2]. 589
Volume 46. number 3
CHEMICAL PHYSICS LETTERS
Fig. 2 demonstrates that both eqs. (3) and (4) are inadequate to describe the pressure dependence of I/lo for A0 and CV, respectively. These comparisons assume that isothermal compression only modifies the viscosity of the solution and that the constants in eqs. (3) and (4) are essentially pressure-independent. However, the experimental results could be fit to eq. (4), if C were assumed to be a pressure-dependent variable. ‘Ihe dependence of the fluorescence yield of substituted methane dyes on fluidity of the environment has been observed by modification of the solvent viscosity by temperature, chemical composition, and now pressure. These dyes may be used as fluorescent probes of the microviscosity of the medium in pressure zxperiments or in other studies, where it is necessary to produce a viscous medium by compression.
590
15 March 1977
References 11) G. Oster and Y. Nishijima, J. Am. Chcm. Sot. 78 (1956) 1581. 121 T?I. Farster and G. Hoffmann. 2. Physik. Chem. NF 75 (1971) 63. rl\ D. Ma&de and M.W. Windsor, Chcm. Phys. Letters 24 ;“I (1974) 144.
r41 M-L. Sheely, Ind. Eng. Chcm. 24 (1932) 1060. ISI P-W. Bridgman. Proc. Am. Awd. Arts Sci. 61 (1926) 57, _ 161 E.V. Zolotykh, Izmerit. Tckhn. 3 (1955) no. 2. I71 P.W. Btidgman. The physics of high prcssurc (Bell, London, 1949).
(81 C.E. Weir. J. Res. Natl. Bur. Std. 53 (1954) 245. PI L.J.E. Hofcr, R.J. Gnbcnstetter and E.O. Wug, J. Am. C&em. Sot. 72 (1950) 203.