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Excited state relaxation of Ru(bpy)32+ at low temperature. Time evolution of the emission quantum yield
Volume 143, number 1
CHEMICAL PHYSICS LETTERS
1 January 1988
EXCITED STATE RELAXATION OF Ru(bpy):+ AT LOW TEMPERATURE. TIME EVOLUTION OF THE EMISSION QUANTUM YIELD
Haeng-Boo KIM, Noboru KITAMURA and Shigeo TAZUKE Research Laboratory of Resources Utilization, Tokyo Institute of Technology, 4259 Nagatsuta, Mdori-ku, Yokohama 227, Japan Received 28 August 1987; in final form 14 October 1987
Decay of emission from the metal-to-l&and charge transfer (MLCT) excited state of Ru(bpy):+ (bpy=2,2’-bipyridine) was studied by single photon counting as well as by nanosecond time-resolved emission spectroscopy in 4 : 1 ethanol-methanol. While the MLCT emission decay of *Ru( bpy):+ could bc fitted by a double exponential function at 125 K, the decay profile was strongly dependent on the monitoring wavelength. This dependence, and a rise component detected for the emission at the lower energy edge of the spectrum, were interpreted on the basis of a time-dependent shift of the emission spectrum.
1. Introduction
Solvent dipole relaxation in excited state decay and electron transfer reactions of both organic and inorganic compounds has been a subject of growing interest [ 11. Previously we reported the time-dependent ( TD) red-shift of the emission from Ru (bpy ) :+ and cis-Ru( bpy ) 2( CN) z around the glass transition temperature (T,, x 130 K for ethanol-methanol (4/l, v/v)) and demonstrated that the phenomena were closely related to solvent dipole relaxation in the metal-to-l&and charge transfer (MLCT) excited state [ 21. Transient absorption spectroscopy for Ru(bpy):+ (T-T absorption) [ 31 and time-resolved emission spectroscopy for Os(phen)(das):+ ( phen = 1, IO-phenanthroline and das= 1,2dimethylarsinobenzene) [ 41 around T, proved the imporant role of solvent dipole relaxation in the excited state decay of these complexes. The TD red-shift of the emission suggests that the emission should exhibit an “apparent” timedependent emission decay, which is inevitably non-exponential. In most cases, however, the MLCT emission lifetimes of Ru( II) and Os( II) complexes at low temperature have been analyzed according to a single exponential function [ 51 and the validity has rarely been discussed. The most pertinent experiment is the determination of the “true” excited state decay profile excluding artifacts caused by the TD
shift of the emission. In the present Letter, we report an elaborate analysis of the decay profile of Ru(bpy):+ emission by means of both nanosecond time-resolved emission spectroscopy using a gated multichannel photodiode array detector and by picosecond time-correlated single photon counting.
2. Experimental Sample preparation has been reported previously [ 21. Emission decay analysis of Ru (bpy ) :’ was per-
formed by picosecond single photon counting using a synchronous cavity-dumped dye laser pumped by a mode-locked cw Nd : YAG laser (Spectral Physics model 3460,3240, 375B, and 344, pulse duration 6 ps, repetition rate 8 or 40 kHz) [6]. Nanosecond time-resolved emission spectroscopy was carried out by the system reported previously [2]. A DG 535 digital delay (Stanford Research) was employed to improve the timing between the laser (Quanta Ray, DCR-1, 355 nm, pulse width x6 ns) and the gate pulser of a multichannel plate/photodiode array detector (Tokyo Instruments Inc./Princeton Instruments Inc.). The gate width of the detector was fixed at = 5 ns throughout this study. The instrumental response of this system was corrected by the use of the known emission spectrum of Ru (bpy ) :+ recorded on a Hitachi MPF-4 spectrofluorometer. All calcu-
0 009-2614/88/$ 03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
77
Volume 143, number 1
CHEMICAL PHYSICS LETTERS
lations were made by a PC-980 1m computer (NEC). Further details of the system will be published in a forthcoming publication [ 71. The temperature was controlled by a liquid nitrogen cryostat (Oxford Instruments Inc. DNl704 and 3 120).
3. Results The emission decay of Ru(bpy)i+ in ethanol-methanol (4/l, v/v) was measured between 80 and 293 K by means of the single photon counting method. The excited state of Ru(bpy):+ decayed single exponentially at 80 K (‘s= 4.44-4.46 ps) and 293 K (7 12-722 ns). At these temperatures, there was no trend of a monitoring wavelength dependence of the lifetime. In the narrow temperature range between 115 and 150 K, on the other hand, the emission decay profile was quite different from that at 80 or 293 K. A typ ical example of the decay pattern at 125 K is shown in fig. 1. The emission decay cannot be fitted to a single exponential function, and also depends strongly on the monitoring frequency, vrn.Namely, a fast de+
9.”
s
C
-4.6
1 January 1988
cay component is observed for urn= 17240 cm-‘, while the emission monitored at lower frequency (i.e. 147 10 cm- ’ ) shows a rise component. The present observation of the rise component for urn= 14710 cm- ’ coincides with the brief report by Ferguson and Krausz [ 81. However, our explanation of this phenomenon is totally different as discussed later. The fitting of the emission decay profiles by a nonlinear least-squares procedure checked by x2 and Durbin-Watson (DW) parameters was successful for a double exponential but not for a triple exponential. The decay times are summarized in table 1. It is noteworthy that the fast decay observed at 17240 cm-’ ( T~( 17240) = 106 ns) does not agree with the rise time component, z2( 147 10) = 136 ns. It is clear that the two-state model represented by the Birks kinetic model is not applicable and that the analytical data given in table 1 are kinetically meaningless [ 91. Both r2( 17240) and 72( 147 10) decrease with temperature and above 150 K, neither fast decay nor fast rise (i.e. single exponential decay) was observed under the present experimental conditions. The results agree very well with the absence of the TD red-shift of the emission above this temperature [ 21. The nonexponential decay of the emission shown in fig. 1 is therefore likely to be the result of the TD red-shift of the emission.
4. Discussion 4.1. Time evolution of the emission quantum yield
0
1
2
3
4
Time / ps
Fig. I. Emission decay profiles of Ru(bpy)$+ in ethanolmethanol (4/l, v/v) at 125 K. Emission was monitored at (a) 17240, (b) 15380,and (c) 147lOcm-I.
78
When the TD shift of emission is observed, the decay profile of the emission intensity ( Zcm(t)) varies with the monitoring wavelength [ 91 so that the excited state decay analysis should be based on the time evolution of the total emission quantum yield, y,,(t). Relative y,,(t) values of Ru(bpy):’ at various delay times were obtained by nanosecond time-resolved emission spectroscopy. The examples at 125 K are shown in fig. 2 together with Zcm(t) at 17240 and 14710 cm- ’ determined under the same instrumental conditions. The emission profiles monitored at 17240 and 147 10 cm- ’ exhibit fast decay and rise, respectively, in the first several hundred nanoseconds, while y,,(t) is a single exponential function with a life-
Volume 143, number 1
CHEMICAL PHYSICS LETTERS
Table 1 Emission decay of*Ru(bpy):+
I January 1988
in ethanol/methanol (4/l, v/v) at 125 K
urn
‘5’
72
(cm-‘)
(us)
(us)
17240 15380 14710
1.50f0.01 (85.4) a) 1.79f0.07 (99.6) 1.85f0.07 (98.3)
0.106 k 0.003 0.057Ik0.010 -0.136f0.010b’
x2
DW
1.374 1.052 1.159
1.683 1.831 1.870
a’ Fraction of the decay component in 96. ‘) Rise time.
time of r,, x 1.8 ps. This value agrees with 7, ( 14710) and z1 (15380) where the emission spectrum is rather flat and almost free from the effect of the TD shift as compared with that around 17240 cm- ‘, It is concluded that the excited state of Ru(bpy):+ decays singk exponentially at 125 K. 4.2. Origin
ofnon-exponential
emission decay
The observed trend that T, and f, determined at urn= 17240 cm- ’ are shorter than the corresponding decay and rise times for the emission at 147 10 cm- ‘, respectively, can be explained by the TD shift of the emission. Since the shape of the emission spectrum of Ru( bpy )$+ did not change appreciably with time [2], we simulated the time evolution of the spectrum based on reXand the TD shift of the emission maximum as shown in fig. 3 [ 71. The (r) and (d) signs inserted in fig. 3 indicate the growing-up and decay of the emission intensity with time, respec-
2-2 0
tively. The emission intensities at 17240 and 15800 cm-’ decay while those around 16500 and 15000 cm-’ exhibit growing-up of the emission intensity (fig. 3a). It is apparent that the fast decay and rise components at the higher and lower energy edges of the individual vibrational bands, respectively, are caused by the TD (low energy) shift of the emission, Indeed, we observed a rise component (196 ns at 125 K) for the emission at the lower energy edge of the 0,O vibrational band (16500 cm-‘) as expected. The temperature dependence of the apparent decay and rise components determined at a fixed wavenumber is also understandable since the rigidity of the medium varies with temperature, as discussed previously [2].
700
Ir, 12
3
0
12
3
4
Time /us
Fig. 2. Time evolution of the emission quantum yield (we,,,, left) and emission intensities (I,,, right) of Ru( bpy):+ at 17240 ( l) and 14710 cm-’ (0) at 125 K. The excitation laser pulse (355 nm) duration and the detector gate width were both ;2:5 ns.
650
I I
15
600
16 17 u X10-3 cm’
X. nm
I
18
Fig. 3. Time-dependent emission spectra of Ru(bpy):+ at 125 K in (a) nanosecond and (b) microsecond time regimes simulated on the basis of ‘I., and the TD shift of the emission maximum. (a) f=O, 40,80, 120,160,200 ns; (b) t=O, 0.5, 1.0, 1.5,2-O, 2.5, 3.0,3.5 ps.
79
Volume 143, number 1
CHEMICAL PHYSICS LETTERS
This is the whole story of the “upparent” extraordinary non-exponential decay of Ru(bpy)$+ emission around TB’Contrary to the report by Ferguson and Krausz that the rise and decay of the emission from *Ru(bpy):+ are attributable to the transition from the charge-delocalized excited state to the charge-localized excited state [ 81, the emission decay dynamics of *Ru( bpy)<+ at low temperature can be explained by the TD red-shift of the emission spectrum relevant to solvent dipole relaxation in the MLCT excited state of Ru( bpy) :’ . The implication of the TD emission shift of Ru(bpy):+ in relation to the current debate on the delocalized/localized de scription of the excited state will be discussed in the forthcoming publication [ 71.
5. Concluding remarks The most important conclusion of the present study is that the excited state lifetime of Ru(bpy) $+ should be determined by the time evolution of v,,(t). The excited state properties of various Ru( II) and Os( II) complexes at low temperature have been studied extensively in recent years [ 5 1. However, the MLCT excited state of all these complexes around T, of the medium will exhibit TD shift of the emission spec-
80
1January 1988
trum. Unless one confirms the absence of time-dependent shift of the emission, an excited state lifetime based on Zem( t) derived at a fixed wavenumber may be an artifact. Therefore, the excited state properties of these complexes reported so far should be reinvestigated on the basis of w,,(t). References [ 1] T. Gennett, D.F. Milner and M.J. Weaver, J. Phys. Chem. 89 (1985) 2787; D. Huppert, H. Kanety and E.M. Kosower, Faraday Discussions Chem. Sot. 74 ( 1982) 161. [2] N. Kitamura, H.-B. Kim, Y. Kawanishi, R. Obata and S. Tazuke, J. Phys. Chem. 90 (1986) 1488. [ 31 S.J. Milder, J.S. Gold and D.S. Kliger, J. Phys. Chem. 90 (1986) 548. [ 41 E. Danielson, R.S. Lumpkin and T.J. Meyer, J. Phys. Chem. 91 (1987) 1305. [ 51 R.S. Lumpkin and T.J. Meyer, J. Phys. Chem. 90 (1986) 5307; F. Barigelletti, A. Juris, V. Balzani, P. Belser and A. von Zelewsky, J. Phys. Chem. 91 (1987) 1095, and references therein. [6] T. Ikeda, B. Lee, S. Kurihara, H. Yamaguchi and S. Tazuke, in preparation. [ 7 ] H.-B. Kim, N. Kitamura and S. Tazuke, in preparation. [ 81J. Ferguson and E. Krausz, Chem. Phys. Letters 127 (1986) 551. [9] J.R. Lakowicz, Principles of fluorescence spectroscopy (Plenum Press, New York, 1984) ch. 8.
CHEMICAL PHYSICS LETTERS
1 January 1988
EXCITED STATE RELAXATION OF Ru(bpy):+ AT LOW TEMPERATURE. TIME EVOLUTION OF THE EMISSION QUANTUM YIELD
Haeng-Boo KIM, Noboru KITAMURA and Shigeo TAZUKE Research Laboratory of Resources Utilization, Tokyo Institute of Technology, 4259 Nagatsuta, Mdori-ku, Yokohama 227, Japan Received 28 August 1987; in final form 14 October 1987
Decay of emission from the metal-to-l&and charge transfer (MLCT) excited state of Ru(bpy):+ (bpy=2,2’-bipyridine) was studied by single photon counting as well as by nanosecond time-resolved emission spectroscopy in 4 : 1 ethanol-methanol. While the MLCT emission decay of *Ru( bpy):+ could bc fitted by a double exponential function at 125 K, the decay profile was strongly dependent on the monitoring wavelength. This dependence, and a rise component detected for the emission at the lower energy edge of the spectrum, were interpreted on the basis of a time-dependent shift of the emission spectrum.
1. Introduction
Solvent dipole relaxation in excited state decay and electron transfer reactions of both organic and inorganic compounds has been a subject of growing interest [ 11. Previously we reported the time-dependent ( TD) red-shift of the emission from Ru (bpy ) :+ and cis-Ru( bpy ) 2( CN) z around the glass transition temperature (T,, x 130 K for ethanol-methanol (4/l, v/v)) and demonstrated that the phenomena were closely related to solvent dipole relaxation in the metal-to-l&and charge transfer (MLCT) excited state [ 21. Transient absorption spectroscopy for Ru(bpy):+ (T-T absorption) [ 31 and time-resolved emission spectroscopy for Os(phen)(das):+ ( phen = 1, IO-phenanthroline and das= 1,2dimethylarsinobenzene) [ 41 around T, proved the imporant role of solvent dipole relaxation in the excited state decay of these complexes. The TD red-shift of the emission suggests that the emission should exhibit an “apparent” timedependent emission decay, which is inevitably non-exponential. In most cases, however, the MLCT emission lifetimes of Ru( II) and Os( II) complexes at low temperature have been analyzed according to a single exponential function [ 51 and the validity has rarely been discussed. The most pertinent experiment is the determination of the “true” excited state decay profile excluding artifacts caused by the TD
shift of the emission. In the present Letter, we report an elaborate analysis of the decay profile of Ru(bpy):+ emission by means of both nanosecond time-resolved emission spectroscopy using a gated multichannel photodiode array detector and by picosecond time-correlated single photon counting.
2. Experimental Sample preparation has been reported previously [ 21. Emission decay analysis of Ru (bpy ) :’ was per-
formed by picosecond single photon counting using a synchronous cavity-dumped dye laser pumped by a mode-locked cw Nd : YAG laser (Spectral Physics model 3460,3240, 375B, and 344, pulse duration 6 ps, repetition rate 8 or 40 kHz) [6]. Nanosecond time-resolved emission spectroscopy was carried out by the system reported previously [2]. A DG 535 digital delay (Stanford Research) was employed to improve the timing between the laser (Quanta Ray, DCR-1, 355 nm, pulse width x6 ns) and the gate pulser of a multichannel plate/photodiode array detector (Tokyo Instruments Inc./Princeton Instruments Inc.). The gate width of the detector was fixed at = 5 ns throughout this study. The instrumental response of this system was corrected by the use of the known emission spectrum of Ru (bpy ) :+ recorded on a Hitachi MPF-4 spectrofluorometer. All calcu-
0 009-2614/88/$ 03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
77
Volume 143, number 1
CHEMICAL PHYSICS LETTERS
lations were made by a PC-980 1m computer (NEC). Further details of the system will be published in a forthcoming publication [ 71. The temperature was controlled by a liquid nitrogen cryostat (Oxford Instruments Inc. DNl704 and 3 120).
3. Results The emission decay of Ru(bpy)i+ in ethanol-methanol (4/l, v/v) was measured between 80 and 293 K by means of the single photon counting method. The excited state of Ru(bpy):+ decayed single exponentially at 80 K (‘s= 4.44-4.46 ps) and 293 K (7 12-722 ns). At these temperatures, there was no trend of a monitoring wavelength dependence of the lifetime. In the narrow temperature range between 115 and 150 K, on the other hand, the emission decay profile was quite different from that at 80 or 293 K. A typ ical example of the decay pattern at 125 K is shown in fig. 1. The emission decay cannot be fitted to a single exponential function, and also depends strongly on the monitoring frequency, vrn.Namely, a fast de+
9.”
s
C
-4.6
1 January 1988
cay component is observed for urn= 17240 cm-‘, while the emission monitored at lower frequency (i.e. 147 10 cm- ’ ) shows a rise component. The present observation of the rise component for urn= 14710 cm- ’ coincides with the brief report by Ferguson and Krausz [ 81. However, our explanation of this phenomenon is totally different as discussed later. The fitting of the emission decay profiles by a nonlinear least-squares procedure checked by x2 and Durbin-Watson (DW) parameters was successful for a double exponential but not for a triple exponential. The decay times are summarized in table 1. It is noteworthy that the fast decay observed at 17240 cm-’ ( T~( 17240) = 106 ns) does not agree with the rise time component, z2( 147 10) = 136 ns. It is clear that the two-state model represented by the Birks kinetic model is not applicable and that the analytical data given in table 1 are kinetically meaningless [ 91. Both r2( 17240) and 72( 147 10) decrease with temperature and above 150 K, neither fast decay nor fast rise (i.e. single exponential decay) was observed under the present experimental conditions. The results agree very well with the absence of the TD red-shift of the emission above this temperature [ 21. The nonexponential decay of the emission shown in fig. 1 is therefore likely to be the result of the TD red-shift of the emission.
4. Discussion 4.1. Time evolution of the emission quantum yield
0
1
2
3
4
Time / ps
Fig. I. Emission decay profiles of Ru(bpy)$+ in ethanolmethanol (4/l, v/v) at 125 K. Emission was monitored at (a) 17240, (b) 15380,and (c) 147lOcm-I.
78
When the TD shift of emission is observed, the decay profile of the emission intensity ( Zcm(t)) varies with the monitoring wavelength [ 91 so that the excited state decay analysis should be based on the time evolution of the total emission quantum yield, y,,(t). Relative y,,(t) values of Ru(bpy):’ at various delay times were obtained by nanosecond time-resolved emission spectroscopy. The examples at 125 K are shown in fig. 2 together with Zcm(t) at 17240 and 14710 cm- ’ determined under the same instrumental conditions. The emission profiles monitored at 17240 and 147 10 cm- ’ exhibit fast decay and rise, respectively, in the first several hundred nanoseconds, while y,,(t) is a single exponential function with a life-
Volume 143, number 1
CHEMICAL PHYSICS LETTERS
Table 1 Emission decay of*Ru(bpy):+
I January 1988
in ethanol/methanol (4/l, v/v) at 125 K
urn
‘5’
72
(cm-‘)
(us)
(us)
17240 15380 14710
1.50f0.01 (85.4) a) 1.79f0.07 (99.6) 1.85f0.07 (98.3)
0.106 k 0.003 0.057Ik0.010 -0.136f0.010b’
x2
DW
1.374 1.052 1.159
1.683 1.831 1.870
a’ Fraction of the decay component in 96. ‘) Rise time.
time of r,, x 1.8 ps. This value agrees with 7, ( 14710) and z1 (15380) where the emission spectrum is rather flat and almost free from the effect of the TD shift as compared with that around 17240 cm- ‘, It is concluded that the excited state of Ru(bpy):+ decays singk exponentially at 125 K. 4.2. Origin
ofnon-exponential
emission decay
The observed trend that T, and f, determined at urn= 17240 cm- ’ are shorter than the corresponding decay and rise times for the emission at 147 10 cm- ‘, respectively, can be explained by the TD shift of the emission. Since the shape of the emission spectrum of Ru( bpy )$+ did not change appreciably with time [2], we simulated the time evolution of the spectrum based on reXand the TD shift of the emission maximum as shown in fig. 3 [ 71. The (r) and (d) signs inserted in fig. 3 indicate the growing-up and decay of the emission intensity with time, respec-
2-2 0
tively. The emission intensities at 17240 and 15800 cm-’ decay while those around 16500 and 15000 cm-’ exhibit growing-up of the emission intensity (fig. 3a). It is apparent that the fast decay and rise components at the higher and lower energy edges of the individual vibrational bands, respectively, are caused by the TD (low energy) shift of the emission, Indeed, we observed a rise component (196 ns at 125 K) for the emission at the lower energy edge of the 0,O vibrational band (16500 cm-‘) as expected. The temperature dependence of the apparent decay and rise components determined at a fixed wavenumber is also understandable since the rigidity of the medium varies with temperature, as discussed previously [2].
700
Ir, 12
3
0
12
3
4
Time /us
Fig. 2. Time evolution of the emission quantum yield (we,,,, left) and emission intensities (I,,, right) of Ru( bpy):+ at 17240 ( l) and 14710 cm-’ (0) at 125 K. The excitation laser pulse (355 nm) duration and the detector gate width were both ;2:5 ns.
650
I I
15
600
16 17 u X10-3 cm’
X. nm
I
18
Fig. 3. Time-dependent emission spectra of Ru(bpy):+ at 125 K in (a) nanosecond and (b) microsecond time regimes simulated on the basis of ‘I., and the TD shift of the emission maximum. (a) f=O, 40,80, 120,160,200 ns; (b) t=O, 0.5, 1.0, 1.5,2-O, 2.5, 3.0,3.5 ps.
79
Volume 143, number 1
CHEMICAL PHYSICS LETTERS
This is the whole story of the “upparent” extraordinary non-exponential decay of Ru(bpy)$+ emission around TB’Contrary to the report by Ferguson and Krausz that the rise and decay of the emission from *Ru(bpy):+ are attributable to the transition from the charge-delocalized excited state to the charge-localized excited state [ 81, the emission decay dynamics of *Ru( bpy)<+ at low temperature can be explained by the TD red-shift of the emission spectrum relevant to solvent dipole relaxation in the MLCT excited state of Ru( bpy) :’ . The implication of the TD emission shift of Ru(bpy):+ in relation to the current debate on the delocalized/localized de scription of the excited state will be discussed in the forthcoming publication [ 71.
5. Concluding remarks The most important conclusion of the present study is that the excited state lifetime of Ru(bpy) $+ should be determined by the time evolution of v,,(t). The excited state properties of various Ru( II) and Os( II) complexes at low temperature have been studied extensively in recent years [ 5 1. However, the MLCT excited state of all these complexes around T, of the medium will exhibit TD shift of the emission spec-
80
1January 1988
trum. Unless one confirms the absence of time-dependent shift of the emission, an excited state lifetime based on Zem( t) derived at a fixed wavenumber may be an artifact. Therefore, the excited state properties of these complexes reported so far should be reinvestigated on the basis of w,,(t). References [ 1] T. Gennett, D.F. Milner and M.J. Weaver, J. Phys. Chem. 89 (1985) 2787; D. Huppert, H. Kanety and E.M. Kosower, Faraday Discussions Chem. Sot. 74 ( 1982) 161. [2] N. Kitamura, H.-B. Kim, Y. Kawanishi, R. Obata and S. Tazuke, J. Phys. Chem. 90 (1986) 1488. [ 31 S.J. Milder, J.S. Gold and D.S. Kliger, J. Phys. Chem. 90 (1986) 548. [ 41 E. Danielson, R.S. Lumpkin and T.J. Meyer, J. Phys. Chem. 91 (1987) 1305. [ 51 R.S. Lumpkin and T.J. Meyer, J. Phys. Chem. 90 (1986) 5307; F. Barigelletti, A. Juris, V. Balzani, P. Belser and A. von Zelewsky, J. Phys. Chem. 91 (1987) 1095, and references therein. [6] T. Ikeda, B. Lee, S. Kurihara, H. Yamaguchi and S. Tazuke, in preparation. [ 7 ] H.-B. Kim, N. Kitamura and S. Tazuke, in preparation. [ 81J. Ferguson and E. Krausz, Chem. Phys. Letters 127 (1986) 551. [9] J.R. Lakowicz, Principles of fluorescence spectroscopy (Plenum Press, New York, 1984) ch. 8.