J. Phys. Chem. 1987, 91, 1095-1098 of bonding is charge-spin delocalization,22 which preserves aromatic character by allowing for intermolecular resonance stabilization. We are currently exploring this conclusion by applying ab initio calculi-ttions to some small aromatic dimer cations. ~~~~~
(22) Milasevich, S. A,; Saichek, K.; Hinchey, L.; England, W. B.; Kovacic, P. J. Am. Chem. SOC.1983, 105, 1088. (23) Levin, R. D.; Lias, S. G. Ionization Potential and Appearance Potential Measurements; National Bureau of Standards; U S . Department of Commerce: Washington, DC, 1971-1981; NSRD/NBS 71.
1095
Acknowledgment. We thank the National Science Foundation for support of M.S.E.-S. through Grant No. CHE8305045. We also thank Dr. M. Kertesz for helpful discussions on the theoretical calculations and Dr. D. E. Martire for his encouragement throughout the course of this work. Registry No. 1, 105991-19-3; 2, 105991-20-6; 3, 106006-62-6; 4, 105991-21-7; 5, 105991-22-8; 6, 105991-23-9; 7, 105991-24-0; 8, 105991-25-1; 9, 105991-26-2; 10, 105991-27-3; 11, 102978-84-7; 12, 75209-07-3; 14, 75209-00-6; 15, 105991-27-3; 17, 105991-28-4; 19, 105991-29-5; 20, 105991-30-8; 22, 105991-31-9; 23, 102978-89-2.
Temperature Dependence of the Ru(bpy),(CN), and R~(bpy),(i-biq)~+Luminescence F. Barigelletti,*'. A. Juris,lS*bV. Balzani,la.bP. Belser,'c and A. von Zelewsky'c Istituto FRAE-CNR, Bologna, Italy, Istituto Chimico "G. Ciamician", University of Bologna, Bologna, Italy, and Institute of Inorganic Chemistry, University of Fribourg, Fribourg, Switzerland (Received: April 24, 1986; In Final Form: October 3, 1986)
The luminescence behavior (emission spectrum and lifetime) of the uncharged cis-Ru(bpy),(CN)2 complex (bpy = 2,2'-bipyridine) has been studied in propionitrile-butyronitrile solution in the temperature range 84-3 10 K and compared with that of the Ru(bpy),(i-biq),+ cation (i-biq = 2,2'-isobiquinoline). For both complexes luminescence originates from triplet Ru bpy metal-to-ligand charge-transfer (3MLCT) excited states, with CN- and i-biq playing the role of "spectator" ligands. The red shift of the emission maximum observed in the temperature range 110-160 K is attributed to relaxation processes related to changes in the solvent matrix viscosity. The changes in emission lifetime with increasing temperature are accounted for by Arrhenius paths, which lead to the population of upper excited states, and solvent matrix effects, which involve either relaxation of metal-ligand vibrational coordinates or rearrangement of the solvation shell. The contributions of these processes to the overall rate constant of radiationless decay have been estimated from an analysis of the In 1 / vs. ~ 1 / T plots. The solvent rearrangement effect is much more important for Ru(bpy),(CN), than for Ru(bpy)2(i-biq)2+. The values of the kinetic parameters of the Arrhenius path that becomes important at high temperature suggest that Ru(bpy),(CN), and Ru(bpy),(i-biq)*+ exemplify two different limiting cases of radiationless decay behavior.
-
Introduction
The photophysical properties of the members of the Ru"polypyridine family are the object of intense investigation2+ because of fundamental and applicative reasons. A number of studies'*'9 have shown that the luminescent metal-to-ligand (1) (a) Istituto FRAE-CNR. (b) University of Bologna. (c) University of Fribourg. (2) Crosby, G. A. Acc. Chem. Res. 1975, 8, 231. (3) Kemp, T. J. Progr. React. Kinet. 1980, 10, 301. (4) DeArmond, M. K. Coord. Chem. Rev. 1981, 36, 325. (5) Kalyanasundaram, K. Coord. Chem. Rev. 1982,46, 159. (6) Watts. R. J. J. Chem. Educ. 1983. 60. 834. (7j Balzani, V.; Juris, A.; Barigelletti; F.: Belser, P.; von Zelewsky, A. Riken Q. 1984, 78, 78. (8) Seddon, E. A.; Seddon, K. R. The Chemistry . of_Ruthenium; Elsevier, Amsterdam, 1984; Chapter 15. (9) Juris, A,; Barigelletti, F.; Campagna, S.; Balzani, V.; Belser, P.; von Zelewsky, A. Coord. Chem. Rev., manuscript in preparation. (10) Van Houten, J.; Watts, R. J. J. Am. Chem. SOC.1976, 98, 4853. (11) Durham, B.; Caspar, J. V. Nagle, J. K.; Meyer, T. J. J. Am. Chem. SOC.1982, 104, 4803. (12) Caspar, J. V.; Meyer, T. J. J . Am. Chem. SOC.1983, 105, 5583. (13) Caspar, J. V.; Meyer, T. J. Inorg. Chem. 1983, 22, 2444. (14) Allen, G. H.; White, R. P.; Rillema, D. P.; Meyer, T. J. J . Am. Chem. SOC.1984, 106, 2613. (15) (a) Wacholtz, W. M.; Auerbach, R. A,; Schmehl, R. H.; Ollino, M.; Cherry, W. R. Inorg. Chem. 1985,24, 1758. (b) Wacholtz, W. F.; Auerbach, R. A,; Schmehl, R. H. Inorg. Chem. 1986, 25, 227. (16) Barigelletti, F.; Juris, A,; Balzani, V.; Belser, P.; von Zelewsky, A. Inorg. Chem. 1983, 22, 3335. (17) Juris, A.; Barigelletti, F.; Balzani, V.; Belser, P.; von Zelewsky, A. Inorg. Chem. 1985, 24, 202. (18) Barigelletti, F.; Belser, P.; von Zelewsky, A.; Juris, A,; Balzani, V. J. Phys. Chem. 1985,89, 3680.
0022-3654/87/2091-lO95$01.50/0
charge-transfer (MLCT) excited states formed upon light excitation deactivate following nonactivated and activated paths in which the solvent plays an important but not yet fully elucidated role. Recent investigations carried out in our laboratory have also shown that the viscosity of the solvent matrix may substantially affect the emission behavior.I6-l9 In particular, the rate of the radiationless decay process is enhanced on passing from a rigid to a fluid matrix. This occurs presumably because of the coming into play of large amplitude (low-frequency) vibrational modes (such as the Ru-N vibrations),18or by the lower energy separation between the excited and ground states due to solvent rearrangements and following an energy gap law dependen~e.'~ The analysis of the 1 / vs. ~ 1 / T plots has allowed in several cases an estimate of the increase in the rate of the radiationless decay process caused by matrix melting. In an attempt to obtain a better understanding of the role played by temperature and the solvent matrix state in the radiationless decay of the MLCT excited states, we have now investigated the temperature dependence of the luminescence properties of the R ~ ( b p y ) ~ ( c and N ) ~R~(bpy),(i-biq)~+ complexes in nitrile solution in the temperature range 84-310 K. Experimental Section Ru(bpy),(CN)22a (bpy = 2,2'-bipyridine) and Ru(bpy),(ibiq),+ (i-biq = 2,2'-isobiquinoline) were prepared as described elsewhere. The experiments were carried out in a mixture of (19) Barigelletti, F.; Juris, A,; Balzani, V.; Belser, P.; von Zelewsky, A. J . Phys. Chem. 1986, 90, 5190. (20) Belser, P.; von Zelewsky, A,; Juris, A,; Barigelletti, F.; Balzani, V. Gazz. Chim. Ital. 1985, 115, 723.
0 1987 American Chemical Society
-
Barigelletti et al.
1096 The Journal of Physical Chemistry, Vol. 91, No. 5, 1987
T, K 180140
T, K
?
P
180 140 I
1 1 1 1
I
I
I
I
I
100 I
1
a
I
100
I 1000/T, K ” Figure 1. Schematic diagram showing the shift of the emission band maximum as a function of the temperature for Ru(bpy),(CN), (0)and Ru(bpy),(i-biq),+ ( 0 ) .The solid line represents a simple connection of the experimental points. The error is estimated to be 650 cm-I.
freshly distilled propionitrile-butyronitrile solution (4:5 v/v). Diluted solutions ( 10-5-104 M) were sealed under vacuum in 1-cm quartz cells after repeated freeze-pump-thaw cycles. The cells were placed inside a modified C600 Thor cryostat equipped with a 3050 Thor temperature controller. The absolute error on the temperature is estimated to be 2 K. The (uncorrected) emission spectra were obtained by a Perkin-Elmer MPF-44B spectrofluorimeter equipped with a Hamamatsu R928 phototube. The emission lifetimes were measured by a modified Applied Photophysics single-photon equipment. The curves obtained were analyzed according to a single exponential decay in agreement with previous treatments which consider thermal equilibration of the states responsible for the luminescent proper tie^.^^^ The single exponential analysis was performed with nonlinear iterative programsz1 and the quality of the fit was assessed by the xz value close to unity and the residuals were regularly distributed along the time axis. Standard iterative nonlinear programs2’ were also employed to extract the parameters for the temperature dependence of the lifetime. Data treatment was carried out with a PDP/11 microcomputer.
4 8 1000/T, K - ’
12
Figure 2. Temperature dependence of the luminescence lifetime, T , for Ru(bpy),(CN), (0)and Ru(bpy),(i-biq),+ ( 0 ) . The error on T is estimated to be 1 8 % . The full line results from the fitting of eq 1 to the experimental points, see text.
range. Starting at 84 K and increasing the temperature gives a smooth, nearly parallel decrease of lifetime in the range 110-140 K. For R U ( ~ ~ ~ ) ~ ( Chowever, N ) , , another stepwise decrease in lifetime occurs in the range 140-160 K. At higher temperature ( T > 260 K) the slope of the 1/ T vs. 1 / T plot is much more negative for R~(bpy),(i-biq)~+ than for Ru(bpy),(CN),. The changes of the (uncorrected) luminescence intensities with changing temperature were quite similar to those of the emission lifetimes.
Discussion Previous investigations have clearly shown that the luminescence emission of both and Ru(bpy)2(i-biq)z+ originates from triplet Ru bpy metal-to-ligand charge-transfer (MLCT) excited states, with CN- and i-biq playing, to a first approximation, the role of “spectator” ligands. The two complexes have approximately the same ligand field strength and no low-lying ligand-to-metal charge-transfer (LMCT) or ligand-centered (LC) Results levels. In spite of these similarities concerning the nature of the The absorption and emission spectra of Ru(bpy)2(CN)2z0~zz~z3 luminescent excited states and the energy ordering of the low and R~(bpy),(i-biq)~+ have been previously reported in various energy levels, Ru(bpy),(CN), and Ru(bpy)z(i-biq)2+are expected solvents. In-the polar solvent (propionitrile-butyronitrile mixture) to exhibit different interaction with the solvent. Consideration used in this work, the uncorrected emission maxima occurred at of the molecular geometry shows that for the uncharged, polar ~ at 581 595 (84 K) and 670 nm (293 K) for R ~ ( b p y ) , ( c N )and Ru(bpy),(CN), complex the charge separation (with consequent (84 K) and 610 nm (293 K) for Ru(bpy)z(i-biq)z+. Figure 1 shows bpy C T excitation generation of a dipole) caused by the Ru the shifts of the band maxima of the two complexes in the temwill reduce the molecular dipole in the excited state with respect perature range 84-310 K. Starting at 84 K, where the matrix to the ground state. In fluid solution, rearrangement of the solvent is frozen, and increasing the temperature gives band maxima for molecules will occur afterward.25 Also for R~(bpy),(i-biq)~+ the both complexes which move to lower energy. As one can see, both Ru bpy excitation will change the molecular dipole. However, complexes show a smooth shift in the range 110-140 K. For in this case it is expected that the permanent 2+ charge will oppose Ru(bpy),(CN),, however, the emission maximum shows an adto a certain extent polar solvent rearrangement. Thus, the posditional, sharp red shift over the narrow temperature interval sibility of rearranging the solvation shell in going from the ground 140-160 K. Above 160 K the emission occurs at near constant to the 3MLCT excited state (or vice versa) is expected to play energy for both complexes. The total amount of red shift in the a more important role for Ru(bpy),(CN), than for Ru(bpy),(i84-310 K range is 1900 and 800 cm-’ for Ru(bpy),(CN), and biq),+. Ru(b$y)z(i-biq)Z+,respectively. Temperature Dependence of the Emission Spectra. The obFigure 2 shows the temperature dependence of the emission served (Figure 1) red shifts of the maxima of the emission spectra lifetime for the two complexes in the same solvent and temperature on increasing temperature can be qualitatively accounted for on the basis of the above discussion. No displacement of the emission maxima can be observed up to -110 K, i.e. until the solvent (21) Bevington, P.R. Data Reduction and Error Analysis for Physical matrix is rigid. In the temperature range 110-140 K the glass Sciences; McGraw-Hill: New York, 1969. matrix begins to soften. Small changes in the internuclear dis(22) (a) Demas, J. N.; Addington, J. W.; Peterson, S. H.; Harris, E. W. J . Phys. Chem. 1977, 81, 1039. (b) Peterson, S. H.; Demas, J. N. J . Am.
-
-
-
Chem. Soc. 1979, 101, 6571. (23) Bartocci, C.; Bignozzi, C. A.; Scandola, F.; Rumin, R.; Courtot, P. Inorg. Chim. Acta 1983.76, L119; Bignozzi, C. A.; Scandola, F. Inorg. Chem. 1984, 23, 1540.
(24) Harrigan, R. W.;Crosby, G. A. J . Chem. Phys. 1973, 59, 3468. (25) Relatively strong interactions can take place between Ru(bpy)Z(CN)2 and solvents which are suitable to give hydrogen bonds.20.22.23
The Journal of Physical Chemistry, Vol. 91, No. 5, 1987 1097
R~(bpy)~(cN and ) ~Ru(bpy),(i-biq),+ Luminescence
TABLE I: Kinetic Parameters for Excited-State Decay Obtained from the Fitting of Eq 1 to the Experimental Results comp1ex
Ru(bPY)2(cN)za R~(bpy)~(i-biq)~+ *
A , , s-'
AEi, cm-I
A2, s-'
2.6 X IO' 4.8 x 105
450 65
3 x 10'0 5 x 1013
AE2, cm-' 2400 . 3850
TBI'
TBY
K 120 126
B1, s-' 8 X IO4 3 x 105
B2, s-' 1.1 x 106
K 158
"Parameters extracted by separate fitting of the results for T < 140 K and T > 140 K;ko = 2.8 X IO5 s-'. bFrom ref 17 and 18,ko = 3.1 X IO5 S-1.
tances (particularly in the low-frequency, large-amplitude Ru-N vibrations) can thus take place, causing a relaxation in the excited state and a consequent red shift (AE 700-800 cm-') of the emission maxima of both complexes. Judging from the behavior of Ru(bpy)2(i-biq)2+(Figure l ) , one finds that this relaxation is complete at T = 140 K, since a further increase in temperature (Le., a further decrease in viscosity) does not cause any additional red shift. For R ~ ( b p y ) , ( c N ) ~however, , an even larger (AE 1000 cm-') relaxation process takes place in the temperature range 140-160 K. We believe that this second relaxation process, not present in Ru(bpy)z(i-biq)2+,is associated with the rearrangement of the solvation sphere which can only take place when the solvent has reached a sufficient fluidity. It is important to note that both the displacement of intramolecular nuclear coordinates and the rearrangement of the solvent molecules can contribute to the localization of the excited electron on a single bpy ligand.'8,26 Temperature Dependence of Emission Lifetimes. In general, the changes of 1/. vs. 1 / T over a large temperature range can be accounted for by the coming into play of additional contributions to the radiationless decay process of the emitting excited state(s) as the temperature increases:
-
-
1 / = ~ k,
+xk,
in the range 140-160 K and is thought to be associated with the reorientation of the solvent molecules, which can only take place when the solvent has acquired the properties of a nonviscous fluid. As one can see from Figure 2 and Table I, for both complexes two Arrhenius terms are needed to explain the temperature dependence of l /. vs. l / T. For each complex, the same values have been obtained for the Al and AEl parameters by fitting separately the data for T < 140 or T > 140, Le. below and above the melting of the solvent matrix. This indicates that such parameters have an intramolecular origin. By analogy with R ~ ( b p y ) , * + , ~this *~?'~ first Arrhenius term is thought to concern the thermal equilibration of MLCT levels, spaced by 450 cm-l in Ru(bpy)z(CN)z and 65 cm-I in Ru(bpy)z(i-biq)2+. As the temperature increases ( T > 250 K) a second Arrhenius term comes into play (Figure 2, Table I). There is a general agreement in the literature that the temperature dependence (at high temperature) of the emission lifetime of Ru(bpy):+ and other Ru(I1) polypyridine complexes is related to an activated surface crossing from the 3MLCT manifold to a 3MC (metal centered) level (eq 4) which undergoes photochemical and/or photophysical deactivation (eq 5). Note that k, represents the sum of all the k
(1)
3MLCT &= 3MC kb
I
In eq 1, ko is a temperature independent term and k, is the rate constant of the ith step which contributes to the decay process. Previous work has shown that the k, terms can take either the form of an Arrhenius e q ~ a t i o n ~ ~ ~ ~ ' " - ' ~ k , = A, exp(-AE,/RT)
(2)
where A, is a frequency factor and AE, an activation energy, or the form of an empirical e q ~ a t i o n ~ ~ - ' ~
k, =
B, + e x p [ C ~ ( l / T - 1/TB,)I
kc
ground state and/or photoproducts
(5)
rate constants of the processes that deactivate ,MC, with the exception of the back surface crossing to 3MLCT. As pointed out by Meyer and c o - ~ o r k e r s , " -the ~ ~ experimental deactivation rate constant that comes into play at high temperature
k2 = Az exp(-AE,/RT)
(6)
can be expressed as follows on the basis of eq 4 and 5:
(3)
which describes a stepwise behavior centered at a certain temperature TB,. In eq 3, C, is a temperature related to the smoothness of the step, and B, is the value attained by k, at T >> TBi. A way in which eq 3 can be derived is shown in the Appendix. This equation is particularly useful to describe the behavior of the system in the glass-fluid transition region of the solvent matrix. The temperature dependence of the emission lifetime of Ru(bp~),(i-biq)~+ can be accounted for by two Arrhenius terms (eq 2) and one stepwise term (eq 3).",'* For Ru(bpy),(CN)* the behavior can only be accounted for (Figure 2) by using two Arrhenius and two stepwise terms. The kinetic parameters derived from the fitting of eq 1 to the experimental results of Figure 2 are shown in Table I. As one can see from Table I, the two complexes exhibit a quite similar stepwise term: for Ru(bpy)*(CN),, TB,= 120 K and B1 = 8 X lo4 s-l; for Ru(bpy)2(i-biq)2+,TB, = 126 K and B , = 3 X lo5 s-l. This term corresponds to the red shift observed in the emission spectra of both complexes in the range 110-140 K (Figure 1) and is thought to be associated with a relaxation of the Ru-N coordinates or small rearrangements of the solvent molecules, such as translational rearrangements, on softening of the solvent matrix. The stepwise term of Ru(bpy)z(CN)z at higher temperature (TB = 158 K, B, = 1.1 X lo6 s-l), not present for Ru(bpy),(i-biq)2+: corresponds to the red shift observed for the emission spectrum ~
3MC
(4)
~~~~~~
(26) Kober, E.M.; Sullivan, B. P.; Meyer, T J. Inorg. Chem. 1984, 23, 2098.
k2
=
ka[kc/(kb
+
kc)]
(7)
Equation 7 can give rise to two limiting cases: (i) When k, >> kb, the decay of 3MC is rapid and eq 7 becomes k2
= k,
(8)
= A , exp(-AE,/RT)-
(9)
It follows that A , exp(-AE,/RT)
In this limit, A2 and AE2 parameters obtained from the fitting correspond to the preexponential factor and activation energy for ,MC surface crossing (Figure 3i). the ,MLCT (ii) When kb >> k,, the decay of 3MC is slow compared to the back surface crossing to 3MLCT. In such a case, the two states are in equilibrium and eq 7 becomes
-
kZ
=
(ka/kb)kc
(10)
Since k,/kb = K = exp(-AE/RT) exp(AS/R), where AE and AS are the internal energy and entropy differences between 3MLCT and ,MC, from eq 6 it follows that A , exp(-AE,/RT)
= k, exp[-AE/RT] exp[AS/R]
(11)
Neglecting the entropic difference between the two states, which is expected to be small, one can rewrite eq 11 as A , exp[-AE2/RT] = k, exp[-AE/RT]
(12)
The meaning of the experimental quantities A2 and AE2 depends on the nature of the processes that contributes to k,. The following limiting cases are of interest:
1098 The Journal of Physical Chemistry, Vol. 91, No. 5, 1987
Barigelletti et al.
-
s-') vibrations whose activation leads to the 3MLCT 3MC surface crossing region (Figure 3, i and ii-1). By contrast, k', can be much smaller because it represents the rate constant of a radiationless transition having a poor Frank-Condon factor (Figure 3ii-2). R ~ ( b p y ) ~( A~2+ 1013-1014s-I, AE2 4000 cm-1)12,16 is thought to belong to case i.11-14,27Several other Ru" polypyridine c o m p l e ~ e s , ' ~including -~~ Ru(bpy)2(i-biq)2+l 7 (Table I), exhibit comparable A2 and AE2 values and are also considered to belong to the same (i) limit. The A2 and AE2 values for Ru(bpy)2(CN)2, however, are quite different (Table I). In particular, the value for A2 is too low to correspond to the frequency factor of a surface crossing process. This suggests that Ru(bpy),(CN)2 represents a case of limit (ii-2). Other examples of such a limiting case have recently been r e p ~ r t e d . ' ~ ,The '~~ different behavior of Ru(bpy),(CN), and Ru(bpy)2(i-biq)2+may be related to the larger stabilization (- 1100 cm-I, Figure 1 ) of the 3MLCT excited state in fluid solution, which is expected to increase the rate of kb thus favoring limit (ii), and to a higher barrier for )MC deactivation.
-
ground state
M-L
u
Acknowledgment. We thank Mr. G. Gubellini for technical assistance. This work was supported by the Italian National Research Council and Minister0 della Pubblica Istruzione and by the Swiss National Science Foundation.
\
ground
ground
(ii-1)
state
Appendix By analogy with dielectric relaxation processes,28 the ability of the solvent molecules to follow the conformational changes of the solute molecules is expressed by a temperature- (viscosity-) dependent rearrangement time
/
V
state
(11-2)
m
B
M-L
TABLE II: Meaning of A and AE3in the Three Limiting Cases Described in the Text (See also Figure 3 ) limitine case A, AE, A, A,
k:
AEa AE AE
-+ AE,
+ AE)/RT]
= B/(l +
TR/TM)
that reduces to k,M = 0 for T R >> i M and k,M = B for i R > kb (ii-1) k,
