- Email: [email protected]
Temperature dependence of fluorescence lifetimes of trans-stilbene
Volume 5 1, number 1
CHEMICAL PHYSICS LETTERS
1 October 1977
TEMPERATURE DEPENDENCE OF FLUORESCENCE LIFETIMES OF TRANSSTILBENE M. SUMITANI, N. NAKASHIMA, K. YOSHIHAJU The Institute for Molecular Science. Okazaki 444. Japan
ma S. NAGAKURA Ilre Institute for Solid Stare Physics, The University of Tokyo, Minato. Tokyo 106, Japan Received 27
June 1977
The temperature dependence of the fluorescence lifetimes of rrulls-stilbene was measured with a picosecond laser and a streak camera. A smooth sigmoidal relation was obtained, quite similar to the temperature dependence of fluorescence quantum yields. The results have clearly shown that the radiative rate is essentially constant in the temperature range from 77 K to 295 K.
1. Introduction Cis-trans geometrical isomerization of stilbene is one of the most fundamental photochemical reactions. Many workers have attempted to understand the mechanism of the direct photoisomerization of stilbenes [I]. The quantum yield of fluorescence (@Jo)increases as the temperature decreases, whereas quantum yield of ‘trans to cis photoisomerization (@J decreases as temperature decreases. The rate of change of both Gf and @t with temperature is greatest at about 200 K. It is most important to know the temperature dependence of the fluorescence lifetime in order to understand the mechanism of this photochemical reaction. In the present work, the dependence of the fluorescence lifetime on the temperature was obtained using a picosecond laser. The large temperature variations of the fluorescence quantum yields are explained by a temperature dependence of the radiationless process. The radiative lifetimes are constant from 77 to 295 K.
2. Experimental The 4th harmonic
(266 nm) of a passively mode-
locked Nds+:YAG laser (1.06 m) excited transstilbene. The fluorescence decays were measured with a Hamamatsu TV streak camera (HTV C979). The streak was digitized by a TV camera/microcomputer system (HTV ClOOO). Due to the short wavelength cut off of the streak camera optics, light with wavelength shorter than 380 nm could not be observed_ The pulse width of the second harmonic (530 nm) was measured to be 30 ps and we can assume that the pulse width of the 4th harmonic is the same or shorter 121. The time resolution of the whoie system at the sweep speed used was = 30 ps. The fluorescence lifetimes (~3 were obtained from the decay curves by an iterative fitting procedure which simultaneously adjusts pre-exponential factor and TV At least five decay curves were obtained for each measurement, and in each case a good fit to a single exponential was found. The trans-stilbene was purified by zone refining and sublimation in the dark. To correlate the published quantum yields of fluorescence Qf) and tram to cis isomerization ($J with the observed fluorescence lifetimes, we employed a deaerated, dilute solution (5 X lO-s M) of frans-stilbene in a 3:2 methyIcyclohexane/ iso-hexane (MCH/IH) solvent. The temperature was controlled with an Oxford Instruments cryostat 183
1 October 1977
CHEMICAL PHYSICS LETTERS
VoIume 5 1. number 1
(model CF204), and the temperature of the sample was measured with a corrected chrome1 vetsus gold cobalt
thermocouple
attached
to the sample
(a)
cell.
3. Results and discussion The fluorescence
decay
curves
of tmns-stiibene
measured at various temperatures are shown in fig. 1, The observed decays were all single exponentials. The fluorescence lifetimes (rf) are plotted in fig. 2a against the temperature_ A smooth sigmoidal relation was obtained, quite similar to the temperature dependence of fluorescence quantum yields obtained by Ma&n and Fischer [33 which are also shown in the same figure. Table 1 lists the observed rf’s with the reported &and r& at different temperatures. The S, radiative (IQ), radiationless (kI), and isomerization (k,) rates are also listed in table l_ These constants
are
plotted as a function of temperature in fig. 2b. As is shown in fig. 2a, the decrease in rf is paralleled by a corresponding decrease in +. Therefore, as seen in fig. 2b, the radiative rate constant is constant at 6 X lo* s-l within experimental error. This agrees well with the value of kr = 5.9 X IO* s-l derived from the integrated absorption spectrum [6]. fir’s and kt’s decrease rapidly with decreasing temperature paralleling each other. From the temperature dependence of the fhrorescence lifetimes, the activated decay process of the fluorescent state has an energy barrier of 3.6 f 0.4 kcal/mole as shown in fig. 3_ This is in agreement with measurements of the activation energy from quantum yields (2-4 kcal/mole in various solvents [I])_ This is also in good agreement with the energy barrier for
T/K 1010
-T
*zii109
.
x
1o8
1
-yf,
,
100 200 K Fig.2. (a) The temperature dependence of the observed fluorescence lifetime (-o-), quanrum yield of fluorescence (-X -), and quantum yield of isomerization (- l -) in MCI-I/ IH [3]. The fluorescence lifetime is normalized to the fluorescence lifetime at 77K; r(T)/r(77 K). The best fit curves are shown in sotid lines. (b) The temperature dependence of the radiative (kf; -o -), radiationless (kI; - X -),and isomerization (k,; -0 -) rate constant in MCH/IH. 3cul
Ti
Table 1 Observed ff uorescence Lifetimes rf in ps, reported quantum yields (@f, @t), and calculated rate parameters (kf, kI, kt in lo9 s-l) of dilute MCH/iH solutions of hms-stilbene at
various temperatures T(K) Y-0 --*\
-“-90K
X--=-193K
--=%.-2()8K
Gg3”” .
I
1000
t/ps Fig. 1. The observed fluorescence decay curves normalized at the maximum intensity. The fluorescence was measured at wavelengths longer than 380 nm.
:84
298 273 253 233 208 193 183 90 77
108 126 214 317 693 1090 1220 1610 1650
0.08 0.09 0.1 0.18 0.5 0.63 0.71 --I.0 -1.0
0.50 0.50 0.49 0.46 0.31 0.23 0.18 0.006 0.0
0.74 0.71 0.47 0.57 0.72 0.58 0.58 0.62 0.6
a) Extrapolated or estimated values in ref. [3 I.
8.52 7.22 4.20 2.58 0.73 0.34 0.24 0.06
4.63 3.97 2.29 1.45 0.45 0.21 0.15 0.05
CHEMICAL PHYSICS LETi-ERS
Volume 5 1, number 1
lO’Ol- *\
,
108 30
L5
I
1 October 1977
measured directly without the need for deconvolution. We did not observe two exponential decays as suggested by the photon counting. The fluorescence decay times monotonically decreased with increasing temperature without showing any sign of two exponential decays. The decay curve was also checked at higher temperature (333 K) where the effect should be enhanced - a single exponential was also obtained. In conclusion our measurements have clearly shown that the radiative rate, k, is essentially constant 6.0 X lo8 s-l in the temperature range from 77 K to 295 K.
60
~l/Tl/10-3
3. Plots of 1/7(T)-l/7(77
K) versus l/T. The activation energy (E,) was calculated by the following equation [I ] ; A exp C--EahTl = 1/7(T)-l/7(77 K), E,= 3.5 ?o.s kciy mole, the frequency factor A = (3 5 1) x 10 t2 il. Fig.
trans+cis photoisomerization (4.1 kcal/mole) in 2and 3.methylpentane mixed solvent [4]_ The observed Tf (108 ps) at room temperature also agreed with the S, lifetime estimated by a quenching experiment (= 100 ps) [S]. Recently, the temperature dependence of fluorescence lifetimes was measured with photon counting equipment [6]; from the variation of Tf with temperature, Birch and Birks concluded that a change in the radiative rate (&) contributed to the change in rf’ Since Tf is only 1.6 ns when it is longest at 77 K, it is necessary to rely heavily on the accuracy of deconvolution procedures when using photon counting methods to measure Tf at higher temperatures_ Using our picosecond laser and a streak camera to make accurate measurements of Tf as a function of temperature, the shortest rf (100 ys at room temperature) could be
Acknowledgement We thank Dr. J.M. Morris of th‘e University of Melbourne for writing the single exponential fitting program used in this work. We are also grateful to Dr. A. Namiki for advice and help with the operation of a cryostat_
References [ 11 J. Saltiel. J.D’Agostino, [2]
[3] [4] [S ] [6]
E.D. Megarity, L. hfetts, K.R. Neuberger, M. Wrighton and 0-C. Zafiriou, Org. Photothem. 3 (1973) 1. R.J. Robbins, 1-R. Harowfield, G-R. Fleming, A.E.\V_ Knight, J.M. Morris and G.W_ Robinson, to be published; private communication_ S. Malkin and E. Fischer, J. Phys. Chem. 68 (1964) 1153. R.H. Dyck and D-S. McClure, J. Chem. Phys. 36 (1962) 2326. J. Saltiel and E-D. Megarity, J. Am. Chem. Sot. 94 (1972) 2742. D.J.S. Birch and J-B. Birks, Chem. Phys Letters 38 (1976) 432.
185
CHEMICAL PHYSICS LETTERS
1 October 1977
TEMPERATURE DEPENDENCE OF FLUORESCENCE LIFETIMES OF TRANSSTILBENE M. SUMITANI, N. NAKASHIMA, K. YOSHIHAJU The Institute for Molecular Science. Okazaki 444. Japan
ma S. NAGAKURA Ilre Institute for Solid Stare Physics, The University of Tokyo, Minato. Tokyo 106, Japan Received 27
June 1977
The temperature dependence of the fluorescence lifetimes of rrulls-stilbene was measured with a picosecond laser and a streak camera. A smooth sigmoidal relation was obtained, quite similar to the temperature dependence of fluorescence quantum yields. The results have clearly shown that the radiative rate is essentially constant in the temperature range from 77 K to 295 K.
1. Introduction Cis-trans geometrical isomerization of stilbene is one of the most fundamental photochemical reactions. Many workers have attempted to understand the mechanism of the direct photoisomerization of stilbenes [I]. The quantum yield of fluorescence (@Jo)increases as the temperature decreases, whereas quantum yield of ‘trans to cis photoisomerization (@J decreases as temperature decreases. The rate of change of both Gf and @t with temperature is greatest at about 200 K. It is most important to know the temperature dependence of the fluorescence lifetime in order to understand the mechanism of this photochemical reaction. In the present work, the dependence of the fluorescence lifetime on the temperature was obtained using a picosecond laser. The large temperature variations of the fluorescence quantum yields are explained by a temperature dependence of the radiationless process. The radiative lifetimes are constant from 77 to 295 K.
2. Experimental The 4th harmonic
(266 nm) of a passively mode-
locked Nds+:YAG laser (1.06 m) excited transstilbene. The fluorescence decays were measured with a Hamamatsu TV streak camera (HTV C979). The streak was digitized by a TV camera/microcomputer system (HTV ClOOO). Due to the short wavelength cut off of the streak camera optics, light with wavelength shorter than 380 nm could not be observed_ The pulse width of the second harmonic (530 nm) was measured to be 30 ps and we can assume that the pulse width of the 4th harmonic is the same or shorter 121. The time resolution of the whoie system at the sweep speed used was = 30 ps. The fluorescence lifetimes (~3 were obtained from the decay curves by an iterative fitting procedure which simultaneously adjusts pre-exponential factor and TV At least five decay curves were obtained for each measurement, and in each case a good fit to a single exponential was found. The trans-stilbene was purified by zone refining and sublimation in the dark. To correlate the published quantum yields of fluorescence Qf) and tram to cis isomerization ($J with the observed fluorescence lifetimes, we employed a deaerated, dilute solution (5 X lO-s M) of frans-stilbene in a 3:2 methyIcyclohexane/ iso-hexane (MCH/IH) solvent. The temperature was controlled with an Oxford Instruments cryostat 183
1 October 1977
CHEMICAL PHYSICS LETTERS
VoIume 5 1. number 1
(model CF204), and the temperature of the sample was measured with a corrected chrome1 vetsus gold cobalt
thermocouple
attached
to the sample
(a)
cell.
3. Results and discussion The fluorescence
decay
curves
of tmns-stiibene
measured at various temperatures are shown in fig. 1, The observed decays were all single exponentials. The fluorescence lifetimes (rf) are plotted in fig. 2a against the temperature_ A smooth sigmoidal relation was obtained, quite similar to the temperature dependence of fluorescence quantum yields obtained by Ma&n and Fischer [33 which are also shown in the same figure. Table 1 lists the observed rf’s with the reported &and r& at different temperatures. The S, radiative (IQ), radiationless (kI), and isomerization (k,) rates are also listed in table l_ These constants
are
plotted as a function of temperature in fig. 2b. As is shown in fig. 2a, the decrease in rf is paralleled by a corresponding decrease in +. Therefore, as seen in fig. 2b, the radiative rate constant is constant at 6 X lo* s-l within experimental error. This agrees well with the value of kr = 5.9 X IO* s-l derived from the integrated absorption spectrum [6]. fir’s and kt’s decrease rapidly with decreasing temperature paralleling each other. From the temperature dependence of the fhrorescence lifetimes, the activated decay process of the fluorescent state has an energy barrier of 3.6 f 0.4 kcal/mole as shown in fig. 3_ This is in agreement with measurements of the activation energy from quantum yields (2-4 kcal/mole in various solvents [I])_ This is also in good agreement with the energy barrier for
T/K 1010
-T
*zii109
.
x
1o8
1
-yf,
,
100 200 K Fig.2. (a) The temperature dependence of the observed fluorescence lifetime (-o-), quanrum yield of fluorescence (-X -), and quantum yield of isomerization (- l -) in MCI-I/ IH [3]. The fluorescence lifetime is normalized to the fluorescence lifetime at 77K; r(T)/r(77 K). The best fit curves are shown in sotid lines. (b) The temperature dependence of the radiative (kf; -o -), radiationless (kI; - X -),and isomerization (k,; -0 -) rate constant in MCH/IH. 3cul
Ti
Table 1 Observed ff uorescence Lifetimes rf in ps, reported quantum yields (@f, @t), and calculated rate parameters (kf, kI, kt in lo9 s-l) of dilute MCH/iH solutions of hms-stilbene at
various temperatures T(K) Y-0 --*\
-“-90K
X--=-193K
--=%.-2()8K
Gg3”” .
I
1000
t/ps Fig. 1. The observed fluorescence decay curves normalized at the maximum intensity. The fluorescence was measured at wavelengths longer than 380 nm.
:84
298 273 253 233 208 193 183 90 77
108 126 214 317 693 1090 1220 1610 1650
0.08 0.09 0.1 0.18 0.5 0.63 0.71 --I.0 -1.0
0.50 0.50 0.49 0.46 0.31 0.23 0.18 0.006 0.0
0.74 0.71 0.47 0.57 0.72 0.58 0.58 0.62 0.6
a) Extrapolated or estimated values in ref. [3 I.
8.52 7.22 4.20 2.58 0.73 0.34 0.24 0.06
4.63 3.97 2.29 1.45 0.45 0.21 0.15 0.05
CHEMICAL PHYSICS LETi-ERS
Volume 5 1, number 1
lO’Ol- *\
,
108 30
L5
I
1 October 1977
measured directly without the need for deconvolution. We did not observe two exponential decays as suggested by the photon counting. The fluorescence decay times monotonically decreased with increasing temperature without showing any sign of two exponential decays. The decay curve was also checked at higher temperature (333 K) where the effect should be enhanced - a single exponential was also obtained. In conclusion our measurements have clearly shown that the radiative rate, k, is essentially constant 6.0 X lo8 s-l in the temperature range from 77 K to 295 K.
60
~l/Tl/10-3
3. Plots of 1/7(T)-l/7(77
K) versus l/T. The activation energy (E,) was calculated by the following equation [I ] ; A exp C--EahTl = 1/7(T)-l/7(77 K), E,= 3.5 ?o.s kciy mole, the frequency factor A = (3 5 1) x 10 t2 il. Fig.
trans+cis photoisomerization (4.1 kcal/mole) in 2and 3.methylpentane mixed solvent [4]_ The observed Tf (108 ps) at room temperature also agreed with the S, lifetime estimated by a quenching experiment (= 100 ps) [S]. Recently, the temperature dependence of fluorescence lifetimes was measured with photon counting equipment [6]; from the variation of Tf with temperature, Birch and Birks concluded that a change in the radiative rate (&) contributed to the change in rf’ Since Tf is only 1.6 ns when it is longest at 77 K, it is necessary to rely heavily on the accuracy of deconvolution procedures when using photon counting methods to measure Tf at higher temperatures_ Using our picosecond laser and a streak camera to make accurate measurements of Tf as a function of temperature, the shortest rf (100 ys at room temperature) could be
Acknowledgement We thank Dr. J.M. Morris of th‘e University of Melbourne for writing the single exponential fitting program used in this work. We are also grateful to Dr. A. Namiki for advice and help with the operation of a cryostat_
References [ 11 J. Saltiel. J.D’Agostino, [2]
[3] [4] [S ] [6]
E.D. Megarity, L. hfetts, K.R. Neuberger, M. Wrighton and 0-C. Zafiriou, Org. Photothem. 3 (1973) 1. R.J. Robbins, 1-R. Harowfield, G-R. Fleming, A.E.\V_ Knight, J.M. Morris and G.W_ Robinson, to be published; private communication_ S. Malkin and E. Fischer, J. Phys. Chem. 68 (1964) 1153. R.H. Dyck and D-S. McClure, J. Chem. Phys. 36 (1962) 2326. J. Saltiel and E-D. Megarity, J. Am. Chem. Sot. 94 (1972) 2742. D.J.S. Birch and J-B. Birks, Chem. Phys Letters 38 (1976) 432.
185