-
997
J. Am. Chem. SOC.1981, 103,997-998
Solvent Isotope Effect on the Quartet Doublet Intersystem Crossing Efficiencies of Cr(bpy)$+ and Cr(phen)$+
Table I. Emission Intensities and Lifetimes of Cr(bpy)," and Crlohenl, '+ in Deaerated H-0-D,O Mixtures at 22 "C compd Cr(bpy),'+
R. Sriram and Morton Z. Hoffman*
I, arb units
'7,ms
@rad (rel)
60.5 46.6 37.3 26.5 18.0 12.6 53.0 45.5 45.0
0.066 0.06 7 0.066 0.066 0.063 0.06 1 0.23 0.20 0.19
1.0 0.77 0.62 0.44 0.32 0.23
0.20 0.40 0.60 0.80 0.99
Department of Chemistry, Boston University Boston, Massachusetts 02215 Cr(phen), '+
Nick Serpone
XD
0.00
0.00 0.50 0.99
Department of Chemistry, Concordia University Montreal, Quebec, Canada H3G lM8
1.0 0.99 1.0
Received July 28, 1980 In the course of our continuing investigation into the effect of solution medium on the photophysics and photochemistry of polypyridyl complexes of Cr(III),z4 we have observed that the intensity of luminescence from the 'E stateSof Cr(bpy),,+ (bpy = 2,2'-bipyridine) is significantly lower in D 2 0 than in H20, although the lifetime of 2E is virtually unchanged. Inasmuch as the intensity of luminescence reflects the population of 2E, the effect indicates that 4Tz, which is the precursor to 'E, is sensitive to the isotopic nature of the solvent. Because of the very short lifetime (- 10 P S )of ~ 4T2 and its nonfluorescent nature in aqueous solution, very few other probes are available for its examination. While there are examples of enhancement of fluorescence yields for rare-earth complexes' and an increase in phosphorescence lifetimes in simple Cr(II1)-ammine complexes* when the solvent is changed from H 2 0 to DzO, this appears to be the first report where the quantum yield of luminescence is reduced by the presence of D 2 0 . Solutions of 1.1 X M C r ( b ~ y ) , ~or+ Cr(phen),,+ (phen = 1,IO-phenanthroline), available as Clod- salts from previous studies? were prepared from concentrated stock solutions in H 2 0 (distilled and passed through a Millipore purification train) to D 2 0mole fractions (xD)from 0.00 to 0.99 using 99.9% D20 (Alfa) which had been distilled under N 2 from K M n 0 4 and BaO. The solutions were deaerated with Ar for 30 min, and all measurements of 'E lifetime ( 2 ~ )and luminescence intensity (0 were made a t 22 f 1 OC. Emission lifetimes (at 727 nm) were measured by using 347-nm excitation from a frequency-doubled ruby laser pulse; emission intensities (at 727 nm) were determined for the same solutions by using a Perkin-Elmer MPF-2A recording spectrofluorimeter (excitation 3 10 nm)I0 equipped with a temperature-controlled cell compartment and a red-sensitive photomultiplier tube. The reproducibility error in the emission decay data, which were strictly first order, was
