Absolute Emission Quantum Yields of Powdered Samples
2229
a the Determination of Absolute Emission Quantum Yields of Powdered
ark SoWrighton,” David S. Ginley, and David L. Morse ~
~
~
a of ~Chemistry, ~ ~ eMassachusetts n t Institute of Technology,Cambridge, Massachusetts 02139
(Received January
IO, 1974; Revised Manuscript Received August 1, 1974)
~ ~ ~ l ~ ccosts a ~ i assisted on by the National Science Foundation
A technique is described for the determination of absolute emission quantum yields of powdered samples using 11conventional scanning emission spectrophotometer. The technique is applied to the determination of the luminescence yields of National Bureau of Standards phosphors, sodium salicylate, tetrahalomanganese(I1) complexes, ClRe(C0)3(1,lO-phenanthroline), R~(2,2’-bipyridine)3~+, and bis(diphosphine) complexes of Rh(I) and Ir(1). Measurement of absolute emission yields involves (1)determining the diffuse reflectance of the sample relative to a nonabsorbing standard at the excitation wavelength, and then (2) measuring the emission of the sample under the same conditions. The quantum yield for emission is then the ratio of the emitted photons to the difference in the number of diffuse reflected photons from the sample and the nonabsorbing standard. The only calibration necessary is to determine the relative detector reiqonse at the excitation and emission wavelengths. The technique gives yields estimated to be f25%, and e validity of the technique has been established by comparison of yields previously obtained for phosphors; by obtaining point-by-point reflectance spectra of powdered samples; and by demonstration that the luminescence quantum yields are independent of the fraction of incident light absorbed by the sample.
Introduction Experimental study of the excited state decay of metal complexes has included largely measurement of the quantum efficiency for chelmical change1 and emission lifetime determination for luminescent complexes.2 The measurement of absolute emission quantum yields3 is, by comparison, a difficult and tedious procedure. Despite the fact that many metal complexes only emit in the solid state, luminescence quantum efficiencies of most molecules have been reported for solutions and are measured relative to a standard under conditions where the sample and standard have the same absorbance. Determination of the luminescence efficiency of powdered samples4 has been restricted mainly to phosphor^.^-^ Determination of absolute rates for radiative decay and nonradiative demy requires both a lifetime and an emission quantum yield determinati0n.l Increasing interest in solid state chemistry, metal complex photochemistry, and investigation of the properties of surfaces has prompted us to seek a rolutine method for the determination of absolute luminescence yields of powdered samples. We now describe a technique for this measurement, which requires only a Conventional scanning emission spectrophotometer. The technique is a reasonable extension of a previously reported method where the intensity of reflected plus emiter i s compared to the intensity of reflected light from a reflectance standard employing a constant response thermopile detector and a filtered mercury lamp excitation source 536
The Technique Luminescence quantum yields of powdered samples can be obtained using a conventional scanning emission spectrophotometer equipped with a front surface emission attachment. The apparatus is schematically shown in Figure
1. The excitation light is incident normal to the sample; some of the light is reflected, some absorbed, and some emitted. For the infinitely thick powdered sample the angular distribution of diffuse reflected light and emitted light is assumed to be the same, ie., obeys Lambert’s law. Theoretical8 and experimentalg support for this assumption is available.1° The same fraction of emitted and reflected light is thus collected by the mirror set at a constant, but arbitrary, angle. In practice the angle of the mirror is adjusted to maximize the emission signal. The light collected by the mirror is analyzed both with respect to relative intensity and spectral distribution. Without changing any parameters, and depending on a constant exciting source intensity, a powdered sample known not to absorb any light is placed in the sample position and the diffuse reflected light analyzed for the sample. A presentation of the required information is shown in Figure 2 for [EtdNIzMnBrd.The luminescence quantum yield is just the ratio of the area under the corrected emission curve to the difference in corrected area under the diffuse reflectance curves for the sample and the nonabsorbing powder. The only prior instrumental calibration necessary is determination of the detector response as a function of energy. Calibration of the relative excitation source intensity as a function of energy enables one to measure the luminescence quantum yield of a powder without requiring a reflectance standard. This is achieved by the following procedure: (1)measure the diffuse reflectance of the sample at an energy where it is known to be nonabsorbing (generally lower than the emission energy); ( 2 ) measure the diffuse reflected and emitted light at a second energy where some absorption occurs. The expected diffuse reflected light intensity a t the second energy (if no absorption had OCcurred) is calculated from the measured value at the first energy and the relative output of the excitation source. The necessary experimental data for a luminescence yield deThe Journal of Physical Chemistry, Vol. 78, No. 22, 1974
2230
M. S.Wrighton, D. S. Ginley. and D.L. Morse E xci tot i o t i Source Mmochromator
i
Mirror2
-
i
11 Silts
Monochromator
qj:;?.-+~
Photomultiplier
l1
Detector
ii e CCI rd e r
Figure 1. Idealized drawing of apparatus for determination of lumi-
W a v e l e n g t h , nm
nescence quantum yields.
Figure 3. Point-by-point reflectance spectrum of pure powdered
[Et4N]2MnBr4; Na2S04 is the reflectance standard. Measurement was made every 10 nm except at 270-290 nm where data were taken every 5 nm. TABLE I: Reflectance Properties of Powders
Relative diffuse reflectance intensity Wavelength, nm
x 10
n
23.0 c---
E n e r g y , cm-I x
IO-^
Figure 2. Raw data necessary for determination of emission quantum yield for [Et4N]eMnBr4:(- -) diffuse reflectance of Na2S04; (-) diffuse re'flectance of [Et4NI2MnBr4;(- - ) emission of
--
[Et4N]2MnBr4re3eorded at ten times the sensitivity of the reflectance
measurements; ( Na2S04.
Y
- ) diffuse
reflectance of [Ef4NIZMnBr4 and
termination are included in Figure 2 and the luminescence yield is found as above. In equation form, the luminescence quantum yield, 4), is given by expression I where E is the area under the cor&J
x2
E no, of photons emitted --no. of photons absorbed (Itstd- Rsmpl) (1)
rected emission curve of the sample and RStd and Rsmplare the corrected areas under the diffuse reflectance curves of the nonabsorbing standard and the sample, respectively, a t the excitation wavelength. For samples which do not absorb in the region of the emission no correction factor is appropriate. If the sample absorbs in the luminescent region then dilution with a nonabsorbing powder can be employed to minimize self-absorption. Otherwise, the luminescence quantum yield i s to be multiplied by a correction factor to take into account self-absorption. The correction factor, C, is approximately given by expression 211 where R' is the re-
IS= 2/(1
+
R')
(2 ) flectivity of the sample in the region of luminescence. The only lirnftaticin of the technique, in principle, is that The Journal of Ph;ysica!Ciiernisfry, Vol. 78. No. 22, 1974
MgO
mirror
powder
300
0.5
0.6
0.6
325 350 375 400 425 450 475
2.8 13.7 35 .O 66.0 91 .o 116 124 108 106 85.0 65 .O 45 .O 28.0 18.0
3.0
2.7 14 .o 36.5 72 .o 86 .O 102 134 109
500
17.0
19.0
Fresh MgOh
525 550 575 600 625 650 675
e ,O
12.4 36.0 67 . 0 90.0 118
126 110 109 87 .O 67.0 45 .0 29 . O 17 . O 8.O
MBr
107
86.0
6's .o
47 .o
30 .o
19 .o 9 .o
Na2S04 0.6 2.8 13.7 35 .O 64 . O
87 .o 109 118
101 98.0 80 .o 61 .O 42 .o 27 .o 17 .O 8 .o
a Relative matimum output from Aminco detector from diffusely reflected excitation by placing the powders in a cuvet in the solid sample accessory. Readings are uncorrected for variation in lamp output and detector sensitivity as a function of wavelength. Prepared by burning Mg in air.
the luminescent material must absorb a measureable fraction of the excitation light. The validity of the technique can be established by (1) comparison of known quantum yields with our determinations; (2) obtaining point-bypoint reflectance spectra to make certain that relative absorbances can be accurately determined; (3) demonstrating that the luminescence yields are independent of the fraction of light absorbed. The results of an investigation of these criteria are detailed below.
Results a. Reflectance Spectra. A point-by-point (every 5 or 10 nm) diffuse reflectance spectrum for the pure solid [Et&I]zMnBrd is shown in Figure 3 using Na2S04 as the reflectance standard. This spectrum typifies similar determinations for [bis(cis - 1,2-bis~d~pheny~phosphino)ethy~ene~ rhodium(I)]+ and -[iridium(I)J+, U~(2,2'-bipyridine)3~+, and ClRe(C0)3(1,10-phenanthroline)in that, spectral band maxima and relative absorption intensities parallel previous transmission and. reflectance measurements for these
2231
Absolute Emission Quantum Yields of Powdered Samples
TABLE 11: Lun;ainest:ence D a t a for Powdered Samples
__--
Sample [Et4NIz.Mn'Brd
[n-Pr4WjsMnUr4 [Et4N]?MnCl4 [n-Bu4NI2Mnl4 [Me4Nl2MnI4 Ir (2=phos)&Ib
R h (2=ph0~)&16 R~(bipy)~Cl,~ C1Re(C0)3(phen) Sodium salicylate NBS 1021 phosphor NBS 1028 phosphor
% light absorbed
Excitation A, nm
Emission max, kcm-1
Emission half-width, cm-'
31 . O
450 450 450 460 470 450 440 430 290 254 460 460 460 460 445 465 475 445 505 450 420 420 420 370 340 254 290 254 290
19.20 19.20 19.20 19.20 19.20 19.20 19.20 19.20 19.20 19.20 19.34 19.15 19.30 18.96 17.98 17.98 17.98 17 .OO 17 .00 16.95 18.38 18.38 18.38 24 35 24.35 19.46 19.46 19.34 19.34
1800 1800 1800 1800 1800 1800 1800 1800 1800 1800 1500 1600 1660 1850 1170 1170 1170 2000 2000 1500 2300 2300 2300 3800 3800 1500 1500 1640 1640
22.7 14.6 47.6 45.7 33.1 23.4 3.3 62.5 36.5 38.4 25.8 46.0 64.5 31.3 18.9 6.8 39.9 13.8 38.6 69.7 62.7 56.0 56.7 90.1 71.2 23.3 92.5 58.3
+ f 25%. 0.57 0.6i 0.68 0.57, 0.50, 0.66 0.60
0.57 0.55 0.58 0.23 0.07 0.56 0.32 0.29 0 .27
0.24 0.23 0.26 0.02
0 .03 0.003 0.0065 0.0050 0.0054 0.62 0.53 0.60 0 .15 0.41
0.21
' For samples which luminesce partially below the region of corrected spectra (lower energy than 16.7 kcm-') we have estimated -the yield by assuming gaussian emission peaks. * 2=phos is cis-1,2-bis(diphenylphosphino)ethylene.c bipy is 2,2'-bipyridine. Phen is l,lO-phenanthroline.
Emiswon A r e a ( a r b i t r a r y units)
Figure 4. Plot OB per cent light absorbed against the relative area under the resulting luminescence curve for [Et4NJ2MnBr4( 0 )bis(cis1,2-bis(diphenylphosphino)e!thyIene)iridium(l) (0),,andCIRe(C0)3(1,lOphenanthroline (A). Variablle per cent light absorbed was achieved either by dilution of the sample with KBr or by varying the excitation wavelength within 0110 absclrption band.
complexes. All of the complexes investigated except the [Et4N]&fnEtr4 required dilution with KBr to diminish the absorptivity to a level where absorption bands could be discerned. Reflectance standards used were common inorganic salts such as I W , Nazl§04, or KBr which are essentially nonabsorbing in the region of interest. Table I gives evidence in the 300-~675-nmregion supporting the notion that the reflectivity of KBr and N a ~ S 0 4are essentially the same as that of fresh MgQ which has extremely high absolute reflectivity. Empirir:ally, we find that particle size has little
bearing on the results. The matching of the reflectance of the standard and the sample a t several wavelengths of lower energy than the emission has been our practical criterion of matching reflectance properties. In general, grinding the samples with mortar and pestle to similar consistency yielded satisfactory results. b. Luminescence Quantum Yields. Emission quantum yields for several powdered samples are given in Table 11. As shown for several cases the emission quantum yield is independent of the per cent light absorbed at a given excitation wavelength. An equivalent presentation of the data is given in Figure 4 where the per cent light absorbed is plotted against the relative luminescence area, and, as required, the plots are linear and extrapolation t o zero light absorbed gives zero luminescence area. The luminescence yields of two National Bureau of Standards phosphors which have been previously measured are included in Table 11. We find 254-nm excitation yields of 0.60 and 0.41 for NBS no. 1021 and no. 1028 phosphors, respectively. Literature values for NBS no. 1021 are 0.45,5 and 0.70,6 and for NBS no. 1028 are Q.685and 0.68.6 Also, sodium salicylate as a pure compound has been measured and has a wavelength-independent emission yield. Previously measured values are 0.50,13 0.64,14and O S g c and we find 0.53 and 0.62 upon 340- and 370-nm excitation, respectively. We have also shown that the relative emission quantum yield is constant for several other data points between 340 and 375 nm. Discussion The results reveal that the proposed technique meets the The Journal of Physicai Chemistry, Voi. 78. No. 22, 1974
M. S. Wrighton, D. S. Ginley, and D,b. Morse
223
criteria outlinrd above to test its validity. Agreement of the values for the phosphor quantum yields is fair, given the large variation in the literature values, and the fact that for the phosphors measured the luminescence yield is extremely wavelength dependent6 which may account for some discrepancy. The agreeiment of the absolute yields for sodium salicylate is extremely good. The 0.99 valuegcfor this compound is incorrect as it is known that the luminescence efficiency of sodium eaiicylate increases by a t least 30%14 l~ upon cooling 1 Q 77"IK. An independent d e t e r m i n a t i ~ n of the emission quantum yield for [Et4N]zMnBr4 upon 460nm excitation yields a value of 0.50 in excellent agreement with the values obtained here evidencing the ability of other workers to obt,sin consistent results. We find that the reproducibitity of a given measurement is likely to be well within 10%. The error in the absolute yields, however, is likely to be much larger for the following reasons: (1) the measurement depends on kdowing the relative response of the detector ab a function of wavelength and we estimate at least f5% and evea worse at the extremes of the calibration range; (2) the absolute reflectivity of the salts used as standards is not unity as we have assumed and the reflectivity of samples in the luminescence region is also not unity; (3) the front surface of the cuvet holding the powder also gives a small amount of scattered light (
