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Picosecond time resolved energy transfer between rhodamine 6G and malachite green
1 June 1978
Volume 56. number 2
PICOSECOND TIME RESOLVED
ENERGY TRANSFER
BETWEEN RHODAMINE 64; AND MALACHITE GREEN G. PORTER and C.J. TRJZDWELL Ikny Fmat&zy Research Labordory of The Royal Institution.
London WIX4BS.
UK
Received 2 &far& 1978
The process of singlet-singlet resonance energy transfh between rhodamiue 6G (donor) and malachite green (acceptor) has been studied with a picosecond laser z streak camera system. Unlike previous investigations, the measurements were conducted in a low viscosity solvent (ethanol) at room temperature. The donor fiuoresceneedecay functionwas found to be in agreementwith thatpredictedby the Fijrs$er theory over a tenfoldr-e of acceptor concentrations(10” M to lo-’ M) and up to a limiting time resolution of 10 ps. An average Ro value of 52.5 A was obtained from the fluorescence decay curves, in reasonable agreement with the value of 48 A calculated from spectroscopic data. .
1. Introduction The transfer of eiectronic excitation energy try resointeractions, between dissimilar molecules in a conde‘nsed phase,
nance dipole-dipole
D*+A+D+A* can occur over intermolecular distances of up to 100 A. The critical transfer distance (R,) is generally used to indicate the strength of the interaction and, by definition, is the distance at which enerlJy transfer has the same probability as the sum of all the other excited state deactivation processes of the donor 11 J . From the Fiirster theory of resonance energy transfer [2] I R:=
(90~
~i0)
P%,D
128 n5n4N
ODw4~&~ s
0
34 Y
dv ,
(1)
where fi2 is an orientation factor, & is the guorescence quantum yield of the donor in the absence of energy transfer, n is the refractive index of the solvent and N is Avogadro’s number_ The spectroscopic functions, F&V) and e*(v), are the spectral distribution of the donor fluorescence and the molar decadic extinction coefficient of the acceptor respectively, and v is the wavenumber. For an isolated donor and acceptor pair, the transfer rate constant (#%I)*_&) can be expressed 278
in terms of the intermolecular distance (R), the fluorescence lifetime of the donor in the absence of energy transfer (q,) and R. [I] I kD*-+k = (l/r&
(4-&
.
(21
The time development of the excited donor population, O*(t), in the presence of a large number of acceptor molecules, is more complex, since there are a wide range of values for the donor : acceptor separation distance (R, ). Consequently, the decay of the excited state donor population is given by [3f I
D*(t) = D*(O) exp I-[ 1 + Zg$
(Ro&,#]
t/q,)
,
where NA is the number of acceptor mofecties surrounding the donor. KN, tends to i&&y, then the time dependent fluorescence decay of the donor, I&), becomes [2---51: IQ@>
=IQ@j
~q&-ftf?b
+
2--&kD
)“‘I
1 ,
(3)
where 7 = CA/Co, Co = 3000/21~~~~NR~and CA is the acceptor concentration. Since all of the parameters can be evaluated experiment&y, a number of authors have investigated the validi@ of the F&ster expression [3$9 J ; resonance energy transfer can occur between the excited sing!et or triplet state of a donor molecule and the singlet ground state or triplet state ofan acceptor, all of these processes have been observed experimentally
Yohxne 56, number 2
1 June 1978
CHEMICAL PHYSICS J&ETTER!S
[3,5-91. Within the limits of the techniques employed, Fijrster theory was found to give an excellent fit to the data. Early kinetic studies were restricted to time scales longer than 5 11sas a result of the duration of the excitation pulses available [S] . The validity of the Forster theory collld not, therefore, be investigated in the subnanosecond region. Picosecond laser techniques now make it possible to time resolve excited state tiecay kinetics on a picosecond time scale, although few studies of resonance energy transfer have been reported [lo--131. The rhodamine 6G (donor) and malachite green (acceptor) system has previously been investigated by Rehm and Eisenthal [11,12] using picosecond absorption spectroscopy. They were able to monitor the decay of the donor ground state anisotropy, induced by a linearly polarised picosecond pulse, which orthogonally polarised probe pulses. The time dependent decay of the ground state anisotropy was in excellent agreement with that predicted by the Forster expression to within 20 ps of excitation. Using a 5 X 104M rhodamine 6G and 5 X 10-3M malachite green solution in glycerol, they obtained an R. value of 53 ir 1 A in comparison with a spectroscopic value of 48 A. A more rigorous investigation of resonance energy transfer can be performed with a picosecond laser: streak camera system 1141, since a complete fluorescence decay curve can be recorded from a single, low intensity, excitation pulse, which precludes many of the errors arising from multiple shot techniques. An accurate study of the donor fluorescence decay kinetics at short times after excitation can therefore be performed with a time resolution of better than 10 ps. In this paper, we report rhe use of picosecond time resolved fluorescence spectroscopy in an investigation of resonance energy transfer between rhodamine 6G and malachite green in ethanolic solution at room temperature.
2. Experimental The donor, laser grade rhodamine 6G (Lambda Physik), was used without further purifcation, whereas the acceptor, malachite green, was recrystallised several times before use. Solutions of rhodamine 6G (lo4 M) and malachite green (see table 1 for concentrations) were prepared in spectroscopic ethanol. A
450
!iOO
550
600
650
700
(rim)
Fig. 1. The absorption (---) and emissian (-) spectra of rhodamine 6G. and the absorption spectrum of malachite green (.._) in ethanol; the structures of the two molecules are also 5hOWIl.
0.02 cm pathlength cuvette had to be used throughout in order to minimise the absorption of the excitation light and the donor fluorescence by the acceptor. The absorption and emission spectra of the donor and acceptor are shown in fig_ 1 along with their molecular structure; the measurement of the donor fluorescence decay was aided by the fact that malachite green has an excited state lifetime of the order of 2 ps and a fluorescence quantum yield of approximately 1O-4 in low viscosity solvents [lS] which obviates the need for selective filtering of the donor fluorescence_ Excitation pulses were generated with a mode-locked, frequency-doubled neodymium:glass laser (530 nm). A single pulse was selected from the centre of the output tram by a Pockels cell electro-optic shutter; the intensity of the pulse was kept below 5 X 1Or4 photons cms2 at the sample to preclude stimulated emission. Fluorescence emitted by the sample was collected at 180° with respect to the excitation beam, passed through a wavelength selection filter (570 nm, Schott OG 570) and focussed onto the slit of an S20 279
Volume 56, number 2
CHEMICAL PHYSICS LETTFZRS
Imacon 600 streak camera (John Hadland (P.I.) Ltd.). A vidicon optical multichannel analyser (OMA 1205 A and B, Princeton Applied Research) digitised the resulting streak trace and stored the data in a 500 channel memory. The linearity of the OMA : streak camera to incident light intensity is better than 23% witbin the range of 30 to 3000 counts in any charmel of the memory, streaks speeds are constant to witbin 1%. Digital data from the memory were displayed on an oscilloscope or transferred to punch tape for analysis. Further details of the Laser system may be found elsewhere [i4]. Since the selected pulse is linearly polarised, the initial portion of the fluorescence decay curves will be distorted by fluorescence depolaisation arising from moIecular rotational diffusion [ 161; this effect can be quite significant even when the detector is not polarised [173. Consequently, an HN 22 (Polaroid) plastic sheet polariser, oriented at 54.7’ with respect to the polarisation of the excitation p&se [IS], was inserted between the sample and the streak camera to compensate for this effect. A number of short-lived fluorescing species have been studied with this a~~gement and all were found to give distortion-free decay curves. Similarly, the fiuorescence decay of rhodamine 6G in ethanol is also exponential under these conditions,
3. Results and discussion The fluorescence lifetime of rhodamiue 6G, in the absence of energy transfer to malachite green, was 4200 t 200 ps in agreement with the value reported by Rehm and Eisenthai [ 1 l] _ Fluorescence emission from a 5 X 10-s M etbanolic solution of malachite green could not be detected by the streak camera at the sensitivity used to record the emission from rhodamine 6G, and was therefore discounted as a potential source of error. To test the FBrster theory of resonance energy transfer, a number of experimentd criteria must be satisfied, namely the duration of the excitation pulse and the extent of rotational and transIational diffusion in the solvent. The theory assumes that the rate of translational diffusion is sIow enough to preclude a significant perturbation of the random distribution of distances be‘tween D* and A during the measurement [5]. In the low viscosity limit, translational diffusion will be rapid and a relatively slow fluorescence decay will be domi280
1 June 1978
nated by Stern-Voimer kinetics [14] _ Consequentiy, previous investigations of resonance energy transfer were performed on molecules in highly viscous media such as plastic ftis [S] , glycerol [lo--121 or low temperature glasses [3]. Siice these media also inhibit molecular rotation [5,16], the orientation factor (~3’) lies between the value for a rapid samphng of all orientations and that for a random, but static, distribution; the exact value for these media is therefore difficult to ascertain 1.51_We have previously shown that rbod~~e 6C in ethanol has a rotation rate of 3.13 X iOmg s-l (Trot = 320 ps) [ 161, which is faster than the average transfer time for all acceptor concentrations except IO-2 M. A similar measurement of the rotation rate of malachite green is precluded by the very short fluorescence lifetime, however the size and charge of the molecule are comparable to those of rhodamine 6G implying that the rotation rates should be similar- The average value of the orientation factor for a dynamically random distribution of orientations, p2 = 2/3, should therefore be a valid description of this system. From the Stokes-Einstein equation 111 and the apparent molecular radius of rhodamine 6G (r = 6 A), obtained from rotational diffusion measurements [16], the diffusion coefficients of both the donor and acceptor are caicdated to be 298 X 10-G cm* s-~. Provided that R,-, b [2(DD +-D&o] l/2, where DD and DA are the diffusion coefficients of the donor and acceptor respectively, the donor fluorescence decay will be governed by Fijrster kinetics [l] ; for the system studied, {2(& + D&Q] U2 = 22.4 A which is small enough to preclude significant changes in the intermolecular distances during energy transfer. Experimental data for the donor fluorescence decay at the three lowest acceptor concentrations are shown in fig. 2 (0 M curve A, 1O-3 M curve B, 2.5 X IO-3 M curve C and 5 X 10-S M curve D). For cktrity, each data point represents the average of 5 OMA channels, however ah of the data were used in the nurnerical analysis_ The time dependence of the donor &IOrescence decay, in the presence of 5 X 10m3 M malachite green, is clearly illustrated by the logarithmic plot of intensity as a function of time shown in fig. 3, This curve is a composite of two sets of data, one set was recorded on a fast streak speed (10 ps resolution, 1 ns full scale) and &e other was recorded on a slower streak speed (100 ps resolution, 10 ns full scale); normalisation of the intensities was performed using the
Volume 56, number 2
CHEMICALPHYSICS LETTERS I-o(t)
IO’
I,(t)
80
1 June 1978
60
IO' 8
6
Fig. 2. The fluorescence decay of rhodamine 6G (10S4 M) in ethanol with (A) 0 M, (B) low3 M, (C) 2.5 X 10e3 M and (D) 5 x 1O-3 M malachite green.
time resolved region common to both sets of data. If the Fbrster expression, eq. (3), describes the donor fluorescence decay function, IQ(~), in the presence of the acceptor, then:
where I&) = IF(O) exp (-f/rD) is the normal fluerescence decay function of the donor in the absence of energy transfer. Consequently, lr! [f~(t>/&(t)] plotted as a function of t i/2 should produce a linear relationship with a slope of -2y~i”~. Fig. 4 shows such a plot for the data illustrated in fig. 3; again the number
4
t
I
I
0
IO
20
1
30
I
I
I
I
40
50
60
70
t plcoseconds 1%
Fig. 4. In[f~(t)/l~(f)] plotted as a function oft I’*_ IQ(~) is given by eq. (3), f~(t) is the fluorescence decay function of rhodamine 6G in the absence of energy transfer and the slope of the curve is -2&*_
of data points has been limited for clarity, but they are not averaged in this instance_ The remaining acceptor concentrations all produced similar curves when the donor fluorescence was plotted in this fashion; values of-y, Co and RO calculated from the slopes of these curves are summarised in table 1. Donor fluorescence decay curves, calculated from the Fiirster expression and the experimental 7 values, are shown as solid lines in figs. 2 and 3. As a result of the streak camera resolution time, some deviation from the theoretical curves is observed within this time region. However, fig. 3 demonstrates that the Fdrster expression can be used to describe the donor fluorescence function to within 10 ps of excitation. An average R. value of 52.5 C OS A was obtained from the results, this is in agreement with the value of 53 i- 1 A reported by Table 1
IO0 1 0
I 05
I IO
I I5
I 20
I 25
I 30
I 35 "aDoI-
I 40
Fig_ 3. A logarithmic plot of fluorescence intensity as a function of time for low4 M rhodamine 6G and 5 X 10m3 M malachite green in ethanol. The solid curve was calculated from eq. (3), see discussion.
CA (1O-3 M)
Rav (A)
Y
1.0 2.5 5.0 6.0 7.5 10.0
73.5 54.1 43.0 40.4 37.5 34.1
0.33 0.83 1.62 1.94 2.36 3.05
* 0.02 f O.C4 it 0.05 it 0.06 f 0.07 f 0.09
co <10-s M)
Ro (A)
3.0 3.0 3.1 3.1 3.2 3.3
53.0 53.0 52.5 52.5 52.0 51.5
f i + f f f
0.1 0.1 0.1 0.1 0.1 0.1
f + + + f f
0.7 0.7 0.5 0.5 0.5 0.5
281
Volume 56, number 2
CHEMICAL PHYSICS LETTERS
and Eisenth~ and compares with the spectroscopic value of 48 A [l l] _ The agreement between the two kinetic values is a further indication that translational diffusion does not distort the kinetics of this energy transfer system at a viscosity of l-2 cP. Recently, it has been reported that the addition of malachite green to the DODCI (3,3’-diethyloxadicarbocyanine iodide) mode-locking solution of a cw purnped rhodamine 6G dye laser increases the stability of the mode-locking process [15,19], whereas malachite green alone does not produce stable mode-locking. We believe 1201 that energy transfer between DODCI and malachite green could explain this observation_ Sicce there is a red-shift of approximately 4C nm between the spectra of the normal DODCI isomer &us = 580 nm, Xn = 605 run) and the photoisomer (&s = 620 run, hn = 645 nm), the overlap with the absorption of malachite green (&,s = 620 run) is greatest for the normai isomer. It seems probable that the R, value for the normal isomer of DODCI ;md malachite green will be much larger than that observed in the rhodamine 6G: malachite green system. Prehminary picosecond time resolved measurements of the fluorescence emission from the normal isomer of DODCI ( lOA M) in ethanolit solution with malachite green (lo4 M) indicated that the DODCI fluorescence lifetime was reduced from 1200 ps to 800 ps [20]. The addition of malachite green to a solution of DODCI appears to provide a rapid deactivation pathway for the excited state of the normal isomer. Further work on this topic, now in progress, should clarify the role of malachite green in the mode-locking of cw dye lasers 1201. R&m
Acknowledgement
overall support of this work and the Ministry of Defence for the award of a Fellowship to CJ.T. We would also like to acknowledge the invaluable assistance of Dr. CM. Keary.
References [ 1] JB. Birks, Photophysics of aromatic molecules (IYileyInterscience, New York, 1970). [2] Th. Fiirster, Discussions Faraday Sot. 27 (1959) 7. [3] J.L. Laporte, Y. Rousset, P. Peretti and P. Ranson, Chem. Phys. Letters 29 (1974) 444. [4] J.B. Birks, J. Phys. Bl (1968) 946. [5] R.G. Bennett, J. Chem. Phys. 41 (1964) 3037. [6] R-G. Bennett, R.P.Schwenker and R E. Kellogg, J. Chem. Phys. 41 (1964) 3040. [7] R.E. Kellogg and R-G. Bennett. J. Chem. Phys. 41 (1964) 3042. [S] R.E. Kellogg, J. Chem. Phys. 41 (1964) 3046. i9] R-G. Bennett, J. Chem. Phys. 41 (1964) 3048. [lo] K-B. Eisenthal, Chem. Phys. Letters 6 (1970) 155. [ 111 D. Rehm and K.B. Eisenthal, Chem. Phys. Letters 9 (1971) 387. [12] K-B. Eisenthal, Accounts Chem. Res. 8 (1975) 118. [ 131 I. Kaplan and J. Jortner, Chem. Phys. Letters 52 (1977) 202. [ 141 M.D. Archer, M.I.C. Ferreira, G. Porter and C J. Trcdwelll, NOW. J. Chim. 1 (1977) 9. [ 151 E P- Ippen, C-V. Shank and A. Bergman, Chem. Phys. Letters 38 (1976) 611. [ 161 G. Porter, PJ. Sadkowski and C J. Tredwell, Chem. Phys. Letters 49 (1977) 416. 1171 E-D. Cehehdk, K.D. IMieIenzand R.A. Velapoldi, J. Res. Nati. Bur. Std. US 79A (1975) 1. 1181 H.E. Lessing and A. von Jena, Chem. Phys. Letters 42 (1976) 213. iI91 E-P. Ippen and C-V- Shank, Appl. Phys. Letters 27 (1975) 488. WI CM. Keary, G. Porter and C-J. Tredweil, unpublished rcSllItS.
We wish to thank the Science Research Council for
282
1 June 1978
Volume 56. number 2
PICOSECOND TIME RESOLVED
ENERGY TRANSFER
BETWEEN RHODAMINE 64; AND MALACHITE GREEN G. PORTER and C.J. TRJZDWELL Ikny Fmat&zy Research Labordory of The Royal Institution.
London WIX4BS.
UK
Received 2 &far& 1978
The process of singlet-singlet resonance energy transfh between rhodamiue 6G (donor) and malachite green (acceptor) has been studied with a picosecond laser z streak camera system. Unlike previous investigations, the measurements were conducted in a low viscosity solvent (ethanol) at room temperature. The donor fiuoresceneedecay functionwas found to be in agreementwith thatpredictedby the Fijrs$er theory over a tenfoldr-e of acceptor concentrations(10” M to lo-’ M) and up to a limiting time resolution of 10 ps. An average Ro value of 52.5 A was obtained from the fluorescence decay curves, in reasonable agreement with the value of 48 A calculated from spectroscopic data. .
1. Introduction The transfer of eiectronic excitation energy try resointeractions, between dissimilar molecules in a conde‘nsed phase,
nance dipole-dipole
D*+A+D+A* can occur over intermolecular distances of up to 100 A. The critical transfer distance (R,) is generally used to indicate the strength of the interaction and, by definition, is the distance at which enerlJy transfer has the same probability as the sum of all the other excited state deactivation processes of the donor 11 J . From the Fiirster theory of resonance energy transfer [2] I R:=
(90~
~i0)
P%,D
128 n5n4N
ODw4~&~ s
0
34 Y
dv ,
(1)
where fi2 is an orientation factor, & is the guorescence quantum yield of the donor in the absence of energy transfer, n is the refractive index of the solvent and N is Avogadro’s number_ The spectroscopic functions, F&V) and e*(v), are the spectral distribution of the donor fluorescence and the molar decadic extinction coefficient of the acceptor respectively, and v is the wavenumber. For an isolated donor and acceptor pair, the transfer rate constant (#%I)*_&) can be expressed 278
in terms of the intermolecular distance (R), the fluorescence lifetime of the donor in the absence of energy transfer (q,) and R. [I] I kD*-+k = (l/r&
(4-&
.
(21
The time development of the excited donor population, O*(t), in the presence of a large number of acceptor molecules, is more complex, since there are a wide range of values for the donor : acceptor separation distance (R, ). Consequently, the decay of the excited state donor population is given by [3f I
D*(t) = D*(O) exp I-[ 1 + Zg$
(Ro&,#]
t/q,)
,
where NA is the number of acceptor mofecties surrounding the donor. KN, tends to i&&y, then the time dependent fluorescence decay of the donor, I&), becomes [2---51: IQ@>
=IQ@j
~q&-ftf?b
+
2--&kD
)“‘I
1 ,
(3)
where 7 = CA/Co, Co = 3000/21~~~~NR~and CA is the acceptor concentration. Since all of the parameters can be evaluated experiment&y, a number of authors have investigated the validi@ of the F&ster expression [3$9 J ; resonance energy transfer can occur between the excited sing!et or triplet state of a donor molecule and the singlet ground state or triplet state ofan acceptor, all of these processes have been observed experimentally
Yohxne 56, number 2
1 June 1978
CHEMICAL PHYSICS J&ETTER!S
[3,5-91. Within the limits of the techniques employed, Fijrster theory was found to give an excellent fit to the data. Early kinetic studies were restricted to time scales longer than 5 11sas a result of the duration of the excitation pulses available [S] . The validity of the Forster theory collld not, therefore, be investigated in the subnanosecond region. Picosecond laser techniques now make it possible to time resolve excited state tiecay kinetics on a picosecond time scale, although few studies of resonance energy transfer have been reported [lo--131. The rhodamine 6G (donor) and malachite green (acceptor) system has previously been investigated by Rehm and Eisenthal [11,12] using picosecond absorption spectroscopy. They were able to monitor the decay of the donor ground state anisotropy, induced by a linearly polarised picosecond pulse, which orthogonally polarised probe pulses. The time dependent decay of the ground state anisotropy was in excellent agreement with that predicted by the Forster expression to within 20 ps of excitation. Using a 5 X 104M rhodamine 6G and 5 X 10-3M malachite green solution in glycerol, they obtained an R. value of 53 ir 1 A in comparison with a spectroscopic value of 48 A. A more rigorous investigation of resonance energy transfer can be performed with a picosecond laser: streak camera system 1141, since a complete fluorescence decay curve can be recorded from a single, low intensity, excitation pulse, which precludes many of the errors arising from multiple shot techniques. An accurate study of the donor fluorescence decay kinetics at short times after excitation can therefore be performed with a time resolution of better than 10 ps. In this paper, we report rhe use of picosecond time resolved fluorescence spectroscopy in an investigation of resonance energy transfer between rhodamine 6G and malachite green in ethanolic solution at room temperature.
2. Experimental The donor, laser grade rhodamine 6G (Lambda Physik), was used without further purifcation, whereas the acceptor, malachite green, was recrystallised several times before use. Solutions of rhodamine 6G (lo4 M) and malachite green (see table 1 for concentrations) were prepared in spectroscopic ethanol. A
450
!iOO
550
600
650
700
(rim)
Fig. 1. The absorption (---) and emissian (-) spectra of rhodamine 6G. and the absorption spectrum of malachite green (.._) in ethanol; the structures of the two molecules are also 5hOWIl.
0.02 cm pathlength cuvette had to be used throughout in order to minimise the absorption of the excitation light and the donor fluorescence by the acceptor. The absorption and emission spectra of the donor and acceptor are shown in fig_ 1 along with their molecular structure; the measurement of the donor fluorescence decay was aided by the fact that malachite green has an excited state lifetime of the order of 2 ps and a fluorescence quantum yield of approximately 1O-4 in low viscosity solvents [lS] which obviates the need for selective filtering of the donor fluorescence_ Excitation pulses were generated with a mode-locked, frequency-doubled neodymium:glass laser (530 nm). A single pulse was selected from the centre of the output tram by a Pockels cell electro-optic shutter; the intensity of the pulse was kept below 5 X 1Or4 photons cms2 at the sample to preclude stimulated emission. Fluorescence emitted by the sample was collected at 180° with respect to the excitation beam, passed through a wavelength selection filter (570 nm, Schott OG 570) and focussed onto the slit of an S20 279
Volume 56, number 2
CHEMICAL PHYSICS LETTFZRS
Imacon 600 streak camera (John Hadland (P.I.) Ltd.). A vidicon optical multichannel analyser (OMA 1205 A and B, Princeton Applied Research) digitised the resulting streak trace and stored the data in a 500 channel memory. The linearity of the OMA : streak camera to incident light intensity is better than 23% witbin the range of 30 to 3000 counts in any charmel of the memory, streaks speeds are constant to witbin 1%. Digital data from the memory were displayed on an oscilloscope or transferred to punch tape for analysis. Further details of the Laser system may be found elsewhere [i4]. Since the selected pulse is linearly polarised, the initial portion of the fluorescence decay curves will be distorted by fluorescence depolaisation arising from moIecular rotational diffusion [ 161; this effect can be quite significant even when the detector is not polarised [173. Consequently, an HN 22 (Polaroid) plastic sheet polariser, oriented at 54.7’ with respect to the polarisation of the excitation p&se [IS], was inserted between the sample and the streak camera to compensate for this effect. A number of short-lived fluorescing species have been studied with this a~~gement and all were found to give distortion-free decay curves. Similarly, the fiuorescence decay of rhodamine 6G in ethanol is also exponential under these conditions,
3. Results and discussion The fluorescence lifetime of rhodamiue 6G, in the absence of energy transfer to malachite green, was 4200 t 200 ps in agreement with the value reported by Rehm and Eisenthai [ 1 l] _ Fluorescence emission from a 5 X 10-s M etbanolic solution of malachite green could not be detected by the streak camera at the sensitivity used to record the emission from rhodamine 6G, and was therefore discounted as a potential source of error. To test the FBrster theory of resonance energy transfer, a number of experimentd criteria must be satisfied, namely the duration of the excitation pulse and the extent of rotational and transIational diffusion in the solvent. The theory assumes that the rate of translational diffusion is sIow enough to preclude a significant perturbation of the random distribution of distances be‘tween D* and A during the measurement [5]. In the low viscosity limit, translational diffusion will be rapid and a relatively slow fluorescence decay will be domi280
1 June 1978
nated by Stern-Voimer kinetics [14] _ Consequentiy, previous investigations of resonance energy transfer were performed on molecules in highly viscous media such as plastic ftis [S] , glycerol [lo--121 or low temperature glasses [3]. Siice these media also inhibit molecular rotation [5,16], the orientation factor (~3’) lies between the value for a rapid samphng of all orientations and that for a random, but static, distribution; the exact value for these media is therefore difficult to ascertain 1.51_We have previously shown that rbod~~e 6C in ethanol has a rotation rate of 3.13 X iOmg s-l (Trot = 320 ps) [ 161, which is faster than the average transfer time for all acceptor concentrations except IO-2 M. A similar measurement of the rotation rate of malachite green is precluded by the very short fluorescence lifetime, however the size and charge of the molecule are comparable to those of rhodamine 6G implying that the rotation rates should be similar- The average value of the orientation factor for a dynamically random distribution of orientations, p2 = 2/3, should therefore be a valid description of this system. From the Stokes-Einstein equation 111 and the apparent molecular radius of rhodamine 6G (r = 6 A), obtained from rotational diffusion measurements [16], the diffusion coefficients of both the donor and acceptor are caicdated to be 298 X 10-G cm* s-~. Provided that R,-, b [2(DD +-D&o] l/2, where DD and DA are the diffusion coefficients of the donor and acceptor respectively, the donor fluorescence decay will be governed by Fijrster kinetics [l] ; for the system studied, {2(& + D&Q] U2 = 22.4 A which is small enough to preclude significant changes in the intermolecular distances during energy transfer. Experimental data for the donor fluorescence decay at the three lowest acceptor concentrations are shown in fig. 2 (0 M curve A, 1O-3 M curve B, 2.5 X IO-3 M curve C and 5 X 10-S M curve D). For cktrity, each data point represents the average of 5 OMA channels, however ah of the data were used in the nurnerical analysis_ The time dependence of the donor &IOrescence decay, in the presence of 5 X 10m3 M malachite green, is clearly illustrated by the logarithmic plot of intensity as a function of time shown in fig. 3, This curve is a composite of two sets of data, one set was recorded on a fast streak speed (10 ps resolution, 1 ns full scale) and &e other was recorded on a slower streak speed (100 ps resolution, 10 ns full scale); normalisation of the intensities was performed using the
Volume 56, number 2
CHEMICALPHYSICS LETTERS I-o(t)
IO’
I,(t)
80
1 June 1978
60
IO' 8
6
Fig. 2. The fluorescence decay of rhodamine 6G (10S4 M) in ethanol with (A) 0 M, (B) low3 M, (C) 2.5 X 10e3 M and (D) 5 x 1O-3 M malachite green.
time resolved region common to both sets of data. If the Fbrster expression, eq. (3), describes the donor fluorescence decay function, IQ(~), in the presence of the acceptor, then:
where I&) = IF(O) exp (-f/rD) is the normal fluerescence decay function of the donor in the absence of energy transfer. Consequently, lr! [f~(t>/&(t)] plotted as a function of t i/2 should produce a linear relationship with a slope of -2y~i”~. Fig. 4 shows such a plot for the data illustrated in fig. 3; again the number
4
t
I
I
0
IO
20
1
30
I
I
I
I
40
50
60
70
t plcoseconds 1%
Fig. 4. In[f~(t)/l~(f)] plotted as a function oft I’*_ IQ(~) is given by eq. (3), f~(t) is the fluorescence decay function of rhodamine 6G in the absence of energy transfer and the slope of the curve is -2&*_
of data points has been limited for clarity, but they are not averaged in this instance_ The remaining acceptor concentrations all produced similar curves when the donor fluorescence was plotted in this fashion; values of-y, Co and RO calculated from the slopes of these curves are summarised in table 1. Donor fluorescence decay curves, calculated from the Fiirster expression and the experimental 7 values, are shown as solid lines in figs. 2 and 3. As a result of the streak camera resolution time, some deviation from the theoretical curves is observed within this time region. However, fig. 3 demonstrates that the Fdrster expression can be used to describe the donor fluorescence function to within 10 ps of excitation. An average R. value of 52.5 C OS A was obtained from the results, this is in agreement with the value of 53 i- 1 A reported by Table 1
IO0 1 0
I 05
I IO
I I5
I 20
I 25
I 30
I 35 "aDoI-
I 40
Fig_ 3. A logarithmic plot of fluorescence intensity as a function of time for low4 M rhodamine 6G and 5 X 10m3 M malachite green in ethanol. The solid curve was calculated from eq. (3), see discussion.
CA (1O-3 M)
Rav (A)
Y
1.0 2.5 5.0 6.0 7.5 10.0
73.5 54.1 43.0 40.4 37.5 34.1
0.33 0.83 1.62 1.94 2.36 3.05
* 0.02 f O.C4 it 0.05 it 0.06 f 0.07 f 0.09
co <10-s M)
Ro (A)
3.0 3.0 3.1 3.1 3.2 3.3
53.0 53.0 52.5 52.5 52.0 51.5
f i + f f f
0.1 0.1 0.1 0.1 0.1 0.1
f + + + f f
0.7 0.7 0.5 0.5 0.5 0.5
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CHEMICAL PHYSICS LETTERS
and Eisenth~ and compares with the spectroscopic value of 48 A [l l] _ The agreement between the two kinetic values is a further indication that translational diffusion does not distort the kinetics of this energy transfer system at a viscosity of l-2 cP. Recently, it has been reported that the addition of malachite green to the DODCI (3,3’-diethyloxadicarbocyanine iodide) mode-locking solution of a cw purnped rhodamine 6G dye laser increases the stability of the mode-locking process [15,19], whereas malachite green alone does not produce stable mode-locking. We believe 1201 that energy transfer between DODCI and malachite green could explain this observation_ Sicce there is a red-shift of approximately 4C nm between the spectra of the normal DODCI isomer &us = 580 nm, Xn = 605 run) and the photoisomer (&s = 620 run, hn = 645 nm), the overlap with the absorption of malachite green (&,s = 620 run) is greatest for the normai isomer. It seems probable that the R, value for the normal isomer of DODCI ;md malachite green will be much larger than that observed in the rhodamine 6G: malachite green system. Prehminary picosecond time resolved measurements of the fluorescence emission from the normal isomer of DODCI ( lOA M) in ethanolit solution with malachite green (lo4 M) indicated that the DODCI fluorescence lifetime was reduced from 1200 ps to 800 ps [20]. The addition of malachite green to a solution of DODCI appears to provide a rapid deactivation pathway for the excited state of the normal isomer. Further work on this topic, now in progress, should clarify the role of malachite green in the mode-locking of cw dye lasers 1201. R&m
Acknowledgement
overall support of this work and the Ministry of Defence for the award of a Fellowship to CJ.T. We would also like to acknowledge the invaluable assistance of Dr. CM. Keary.
References [ 1] JB. Birks, Photophysics of aromatic molecules (IYileyInterscience, New York, 1970). [2] Th. Fiirster, Discussions Faraday Sot. 27 (1959) 7. [3] J.L. Laporte, Y. Rousset, P. Peretti and P. Ranson, Chem. Phys. Letters 29 (1974) 444. [4] J.B. Birks, J. Phys. Bl (1968) 946. [5] R.G. Bennett, J. Chem. Phys. 41 (1964) 3037. [6] R-G. Bennett, R.P.Schwenker and R E. Kellogg, J. Chem. Phys. 41 (1964) 3040. [7] R.E. Kellogg and R-G. Bennett. J. Chem. Phys. 41 (1964) 3042. [S] R.E. Kellogg, J. Chem. Phys. 41 (1964) 3046. i9] R-G. Bennett, J. Chem. Phys. 41 (1964) 3048. [lo] K-B. Eisenthal, Chem. Phys. Letters 6 (1970) 155. [ 111 D. Rehm and K.B. Eisenthal, Chem. Phys. Letters 9 (1971) 387. [12] K-B. Eisenthal, Accounts Chem. Res. 8 (1975) 118. [ 131 I. Kaplan and J. Jortner, Chem. Phys. Letters 52 (1977) 202. [ 141 M.D. Archer, M.I.C. Ferreira, G. Porter and C J. Trcdwelll, NOW. J. Chim. 1 (1977) 9. [ 151 E P- Ippen, C-V. Shank and A. Bergman, Chem. Phys. Letters 38 (1976) 611. [ 161 G. Porter, PJ. Sadkowski and C J. Tredwell, Chem. Phys. Letters 49 (1977) 416. 1171 E-D. Cehehdk, K.D. IMieIenzand R.A. Velapoldi, J. Res. Nati. Bur. Std. US 79A (1975) 1. 1181 H.E. Lessing and A. von Jena, Chem. Phys. Letters 42 (1976) 213. iI91 E-P. Ippen and C-V- Shank, Appl. Phys. Letters 27 (1975) 488. WI CM. Keary, G. Porter and C-J. Tredweil, unpublished rcSllItS.
We wish to thank the Science Research Council for
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1 June 1978