Rise Times of Rhodium(II1) Complexes
The Journal of Physical Chemistry, Vol. 83, No, 23, 1979 2991
A New Approach to the Measurement of Luminescence Rise Times. A Study of the Rise Times of Rhodium(II1) Complexes Steven H. Peterson,? J. N. Demas," Department of Chemistry, University of Virginia, Chariottesville, Virginia 2290 1
T. Kennelly, H. Gafney," Department of Chemistry, Queens Coi/ege, Fhshing, New York 11367
and D. P. Novak" Soltex Polymer Corporation, P.O. Box 27328, Houston, Texas 77027 (Received February 1, 1979; Revised Manuscript Received July 9, 1979)
A new approach to the measurement of luminescence rise times is described which eliminates the problems associated with different transit times of the photomultiplier for different wavelengths. The approach is based on a wavelength shifter with a very long lifetime. The method and a suitable wavelength shifter are described. The technique is used to study the rise times of photochemically important cis-[RhL,(XY)]+complexes where L = 2,2'-bipyridine, 1,lO-phenanthroline, or ethylenediamine and X, Y = C1-, Br-. In the best cases, with a 10-ns excitation pulse, the method shows rise times to be 50.1 ns. Relaxation pathways and photochemical implications are discussed.
There is currently great interest in the photochemical and luminescence properties of transition metal complexes.ld Fundamental to understanding and practical utilization of these processes in such areas as solar energy conversion and laser technology is knowledge of the paths and rates of intramolecular energy degradation. This information has proved exceedingly elusive. Luminescence rise time measurements are potentially a very powerful tool for directly studying intramolecular relaxation phenomena of the following type: D hv **D (1.4)
+
**D *D 2D
-+
*D
(1B)
+ hv and A
(IC) where D is a luminescent species, **D is the species in an initially excited upper excited state, and *D is the thermally equilibrated excited state from which emission arises. T , is the rise time associated with relaxation to the emitting level, and 7 D is the decay time of the emitting level. In principle, T , is not a simple decay time, but the available data rarely justify a more complex treatment. In practice, T , would be measured by exciting the sample with a short optical pulse and monitoring the grow-in of the luminescence of *D. The use of luminescence rise times to measure 7, has not been completely satisfactory. Initially, workers using the luminescence risetime method reported microsecond rise times for Cr(II1) complexe~,~ but other workers using the same technique showed these measurements to be in error; the true values were 110-100 Picosecond absorption techniques have now shown Cr(II1) complexes to have 7;s in the low picosecond range.g We were particularly intrigued by the report of Ohashi et al.lOaon Rh(II1) complexes. These authors reported that solid samples of cis-[Rh(bpy),X,]X exhibited 7,'s of 350-630 ns where bpy = 2,2'-bipyridine and X = C1- or Br-. + Westinghouse Electric Corp., Research and Development Center, 1310 Beulah Rd., Pittsburg, PA 15235.
0022-3654/79/2083-2991$01 .OO/O
These results were interpreted as a hindered relaxation from a ligand-localized 3(7r-71.*) state to the emitting metal localized 3(d-d) state.loa Rhodium(II1) complexes have played a central role in the understanding of the photochemical and photophysical processes of d6 complexes.ll2 The reported slow rise times, if correct, have important and wide-ranging ramifications in the interpretation, and practical utilization, of transition metal complexes. We have carefully reexamined the claimed long 7,'s of the [Rh(bpy)zXz]Xspecies as well as the analogous phen (1,lO-phenanthroline) and similar cisbis(ethy1enediamine) samples. Our results conclusively show that the rise times of model Rh(II1) complexes are subnanosecond or at least three orders of magnitude faster than had been believed. A private communicationlobfrom one of the authors indicates that they have independently confirmed this original error (vida infra).ll To measure the exceedingly fast rise time of these red emitting complexes with our existing equipment, we have been forced to develop a new methodology for photomultiplier artifacts. Our approach is based on a long-lived luminescent wavelength shifter and eliminates the wavelength dependent time shift of the photomultiplier detector and, in our case, its total insensitivity to the excitation pulse. Implementation of the method is described, and the method is shown to be suitable for subnanosecond rise times even when a 10-ns excitation pulse is used.
Experimental Section All complexes were prepared by standard techniquedl and were carefully recrystallized from water. Where it was possible to check, emission spectra and decay times agreed well with the literature values. The excitation setup for emission measurements on solid samples is shown in Figure 1. All samples were held in sealed cylindrical cuvets. The traditional 90° arrangement was unsatisfactory because of the reduced sensitivity and irreproducibility of sample placement; much of the sample was excited on the backside of the sample which is not transparent to its own emission. These problems were solved by making observations at the front surfaces with 0 1979 American Chemical Society
2992
The Journal of Physical Chemistry, VoL 83, No. 23, 1979
Peterson et al.
TABLE I: Luminescence Decay Parameters of Rhodium( 111) Complexes mean decay time,a ps
__
complex
295 K
[R~(~PY),C~,IC~ [ R W P YI2Br,lBr [ Rh(phen),Cl,]Cl [ Rh(phen),Br, ]Br [ Rh (en ) 2 Br 2 1N 0 7 [ Rh(en),BrCl]ClO, [ Rh(en),C12]C10, a
Accuracy is +5%.
11.4 (11.6b) 3.5 (2.4b) 6.4 7.5 4.06 2.19 0.91
Taken from ref loa.
rise time, ns
77 K 46 (45.2,b 46.9') 25 (26.5,* 2 7 . 3 ' ~ ~ ) 34 (36.gC) 3 5 (20.6C,d) 26 20 12.8
' Taken from ref
14.
295 K
77 K
(350b) G0.3 ( 3 4 0 b )
