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J. Phys. Chem. 1982, 86, 4141-4143
is involved. Atmospheric nitrogen which covers the newly created surfaces of mobile cracks is also independently excited. The absence of nitrogen emission lines in the TL spectra of triphenylamine, triphenylphosphine, and triphenylarsine cannot be as readily explained. In these crystals, either energy transfer from nitrogen to the crystal quenches the nitrogen emission, or only the crystal is mechanically excited. Firm conclusions cannot yet be drawn. Mechanism. The TL excitation of piezoelectric crystals can be understood in terms of creation of oppositely charged surfaces on a propagating crack?JOgn It has been shown that the piezoelectric field around the tip of the mobile crack is sufficient to cause the nitrogen emission TL.3 The intense electric field near the tip of the mobile crack may also cause molecular excitation by several mechanisms including electron impact and recombination of charge carriers. The latter mechanism is responsible for the electroluminescence of molecular crystals which occurs at fields much iower than the estimated electric field near the tip of the mobile cracks. It is seen from Table I that some of the nonpiezoelectric crystals including triphenylphosphine, triphenylarsine, and chlorotriphenylmethane are triboluminescent. No piezoelectric charging is expected during the deformation of pure single crystals of these materials. However, trace amounts of randomly oriented impurity sites or defect sites ~~
~
~
~~~
~
~
(27) Vernadsky, W. J. Bull. Acad. Sci. St. Petersbourg 1910,4, 1037 (cited in ref 3).
exist. When a crack passes through such sites, local electric fields could be produced which could excite TL at these sites even though no macroscopic piezoelectrification is produced during the deformation of the crystals. Our observation that the nonpiezoelectric triboluminescent crystal lose their TL intensity faster than piezoelectric crystals is consistent with the explanation based on the production of defects during crystal growth. The relative intensities of TL are probably related to both the magnitude of the piezoelectric fields near the tip of the mobile cracks and to the photoluminescence quantum yields of the crystals. The electric fields will determine the nuber of excited molecules and the relative rates of the radiative and nonradiative transitions will determine the luminescence efficiencies. The electric field between the surfaces will depend on the charge density on the surfaces which differ from crystal to crystal due to the combined effects of different piezoelectric constants, charge leakages, directions of fracture propagation, and fracture stress. Because of the complexities of these processes, quantitative treatments of TL intensity cannot yet be made.
Acknowledgment. B.P.C. gratefully acknowledges the Ministry of Education and Social Welfare, Government of India, for the award of a National Scholarship for Postdoctoral Research. The support of the US. Army Research Office, Durham, and the Camille and Henry Dreyfus Teacher-Scholar Award (J.I.Z.) are gratefully acknowledged.
Intersystem Crossing Efficiency and Doublet Photochemical Quantum Yield in [ W e n ) 3 I 3+ A. D. Klrk' and M. A. Ramp1 Scandola Department of Chemistry, University of Victoria, Victoria, British Columbia, Canada V8W 2Y2, and Centro di Studio sulk Fotochimica e Reactivita degli Stati Eccitati dei Composti di Coordinazione del C.N.R., Istituto Chlmico dell'llnlversita, Ferrara, Italy (Received: January 19, 1982; I n Final Form: June 22, 1982)
The intersystem crossing efficiency in Cr(en)33+has been measured to be 0.68 fO.10 by comparison of the phosphorescence yield on quartet and doublet excitation. The photochemical quantum yield is, within the experimental error of *15%, the same at the two excitation wavelengths. The results allow modeling of the quenchable photochemistry either by direct doublet reaction or by reverse intersystem crossing and quartet reaction. The reasons why the latter is preferred are discussed.
Introduction A problem that has been the focal concern of a number of recent has been the route of the quenchable (slow) fraction of the photochemical reactivity of Cr(en),3+ and similar molecules. We report here the results of two experiments that bear on this and on the literature that direct doublet photochemistry must occur. The general kinetic scheme relevant to the situation of two potentially reactive interconverting states, one of which
emits, is shown in Figure 1. Steady-state analysis leads to the following expressions for the important quantum yields or yield ratios [(l- qpkc) may be a nonsteady-state branching ratio]: (1) b(RXn)Q,fWt= (1- flpisc)fllqr = 0.11
(1) R. Ballardini, G.Varani, H. F. Wagestian, L. Moggi, and V. Balzani, J. Phys. Chem., 77, 2947 (1973). (2) A. D. Kirk, Coord. Chem. Reu., 39,225 (1981). (3) R. Fukuda, R. T. Walters, H. Maecke, and A. W. Adamson, J. Phys. Chem., 83, 2097 (1979). (4) A. D.Kirk, J.Phys. Chem., 85,3205 (1981), and supplementary material. (5) A. W. Adamson, R. C. Fukada, and R. Tom Walters, J . Phys. Chem., 85, 3206 (198l), and supplementary material.
-dJ(p)Q - - 4(Rxn)Q,slow = flpisc + (1- ?pisc))7isc
0022-3654l82f 2086-4141$0l.25/0
4 (RXn)Q,dow = (7pi.c
+ [1- ?pisclflisc)()7dr + flriacvqr) = 0.26 (2) 1 - viscqrisc
~ P ) D+(Rxn)D
=
flD
(3)
Here the q's are the fractional efficiencies of the processes of Figure 1 while the (1- ~ ~ term ~allows~ for q multiple passes through the doublet state. 4(Rxn)Q,fWt, $(RXn)Q,slow, @(Rxn)Dare the quantum yields for fast 0 1982 American Chemical Society
~
4142
The Journal of Physical Chemistry, Vol. 86, No. 21, 1982
1
D'loss
-gs Flgure 1. Exclted-state processes for a Cr(I11) Complex: v,, efflclency of process I; isc, intersystem aosslng; pisc, prompt Lsc (Le.,competithre with vibrational equilibration); rlsc, reverse isc crossing; qr, quartet reaction; dr, doublet reaction.
Kirk and Scandoia
For both wavelengths of excitation 3-pm filtered solutions of known absorbancies in the range 0.4-0.1 (4-cm cell, Unicam SP8-400 spectrophotometer) were flowed at the rates required to maximize the emission signals. At both wavelengths of excitation it was shown that the emission signal was linear in excitation beam power, and great care was taken to ensure that the two beams traversed the same path through the flow cell. This was confirmed visually, and by the fact that the emission signal maximum occurred at both wavelengths for very similar adjustments of the monochromator entrance optics. The zero for the emission signal excited at 668.9 nm was obtained by flowing a like solution of C r ( e r ~ ) ~quenched ~+, by OH-, to allow for any 668.9-nm radiation scatter. The zero thus obtained was very little different from that for water showing that scatter was not a problem. A plot of emission signal per milliwatt excitation power vs. absorbance over the range 0.4-0.2 gave two parallel curves such that for the two wavelengths P(emission)M8.9 mW-'/P(emis~ion)~~~.~ mW-' = 1.91 f 0.04 at all points in the range. A scan of the emission spectra with 514.5- and 668.9-nm excitation showed the emission spectrum to be the same at the two wavelengths, supporting the assumption that the fractional efficiency of emission collection was the same, and can be represented by a simple proportionality constant. Then correcting for the greater number of photons per milliwatt at 668.9 nm P(emission)668,g0~ 668.9P(abs, mW) mW) P ( e m i s s i ~ n )a~ 514.5qg(abs, ~~,~
(unquenchable) and slow (quenchable) photoaquation on quartet irradiation and for photoaquation on doublet irradiation, respectively. $(P)Qand $(P), are the phosphorescence yields on quartet and doublet irradiation. The equation for $(Rxn)Qglow given earlier4 was in error. The reported overall photochemical yield for Wen):+ is 0.376 with 68% l3,' being quenchable, allowing the division of the total quantum yield shown in (1)and (2). The literature claim3s5 that the photochemistry of this system is best described by a doublet fractional population of 0.3, negligible reverse intersystem crossing, and reaction of quartet and doublet states with efficiencies of 0.16 and 0.87 has, in part, prompted this study. Important information relatipg to C ~ - ( e n ) may ~ ~ + be obtained by measurement of the efficiency of doublet population on quartet irradiation, (f in ref 3 and 5) and then, for absorbance matched conditions by measurement of the chemical reaction quantum yield P(emission)M8,gmW-' 668.9 for irradiation directly in the doublet. The former we have =-- 1.91 f 0.04 measured by comparison of the phosphorescence yield for P ( e m i ~ s i o n ) ~mW-' ,~~, 514.5~~ quartet and doublet excitation. Regarding both experihence TD = 0.68 f 0.10. menta it should be noted that the doublet absorption in Cr(en):+ is well separated from the quartet e x ~ i t a t i o n . ~ ~ ~ In view of the absolute nature of the intensity calibrations there is an uncertainty of f10-15% in this new value, The emission measurements were carried out with a as indicated. This value for the total intersystem crossing 4-cm path length, 3-mm i.d. flow cell, in the manner deyield is in excellent agreement with, and independently scribed in detail earlier.'" The monochromator used, confirms, a value of 0.71 obtained earlier12by comparison however, was a Ramanor HG2S, with 0.8-mm slit settings. of sensitized and direct emission yields for Cr(en)?+. This enabled measurements of the emission intensity at The photochemical quantum yield was measured after 14 925 cm-' without significant interference from the expansion of the beam to a diameter of 5 mm. Solutions scattered excitation light. were 2 X M in HC104,3 pm filtered (in some runs 0.2 Phosphorescence was measured under the following pm), and at 668.9 nm, 0.1-0.3 M in complex. Solution conditions: excitation by about 50 mW of 514.5-nm (Ar absorbances were measured with a Unicam SP8-400 laser, Coherent Radiation CR6) or 668.9-nm radiation spectrophotometer and the calculated fractions of absorbed (Coherent Radiation Model 590 dye laser, rhodamine 640 light were in good agreement with direct measurements dye), the latter wavelength being the maximum of the with the power meter. doublet absorption feature of Cr(en)t+. The irradiation Solutions were irradiated (30-60 mW, 40-200 s) at 21-22 powers were measured with a Coherent Radiation Model "C in 1- or 4-cm spectrophotometer cells 4nd an Ingold 210 power meter (specified as f5% flat response 300 nmLOT electrode interfaced to a PDP-11 computer was used 30 pm), calibrated in terms of a bolometer similar to that for pH determinations, giving readout and recording of the described by Wegner and Adamson." This showed the millivolt values at 10-s intervals. At the high complex power meter readings (mW) at the two wavelengths to be concentrations required at 668.9 nm, electrode potential equal within experimental error (10%). Reineckate" and changes of up to 35 mV, slow equilibration (5-10 min), and ferrioxalate actinometry at 515 nm also gave results within hysteresis effects were seen. The effects were such as to 15% of the power meter values; the latter are more congive spurious quantum yields (usually high) with the ApH venient and reliable. method. These problems were solved by using 10-pL additions of standard acid and base to cycle the electrode to (6)W.Geiss and H. L. Schlaefer, 2. Phys. Chem. (Frankfurt am Main), 66, 107 (1969). correct response prior to photolysis. AH+ values for (7)W.L. Waltz, R. T. Walters, R. J. Woods, and J. Lilie, Inorg. Chim. photolysis were then based on back-titrations with Acta, 45,L153 (1980). standard acid to the original pH value; after electrode (8)G. L. Hilmes, H. G. Brittain, and F. S.Richarson, Znorg. Chem., 16,528 (1977). cycling these were within 15% of the values based on the (9)M.C. Cimolino and R. G. Linck, Znorg. Chem., 20, 3499 (1981). (10)A. D. Kirk and G. B. Porter, J. Phys. Chem., 84, 887 (1980). (11)E.E. Wegner and A. W. Adamson, J,Am. Chem. SOC.,88,394 (1966).
(12)F.Bolletta, M. Maestri, and V. Balzani, J. Phys. Chem., 80, 2499 (1976).
[Cr(en),]
'+ Phosphorescence
The Journal of Physical Chemistry, Vol. 86, No. 21, 1982 4143
ApH method. No increased post-photolysis pH changes were observed and the small corrections 1 3 % for concurrent thermal reactions were applied by using the observed rate constant of 5.9 x s-l for the thermal reaction under the conditions of these experiments. An advantage of the high concentration of complex required at 668.9 nm was that several runs could be carried out on the same solution, while remaining below 1% total decomposition. Both for 0.3 M solutions in a l-cm cell and 0.1 M solutions in a 4-cm cell, and excitation powers of 20-60 mW at 668.9 nm, consistent values of the photochemical quantum yield of 0.45 f 0.05 (9 measurements) were obtained based on the power meter readings, corrected for reflections, etc. Since there is a 10% uncertainty in the absolute power calibration, this result is consistent with the published value of 0.37 for quartet irradiation. In view of the increased uncertainties due to the absolute power calibration, we measured $(514.5 nm) similarly and obtained 0.42 f 0.02 (6 measurements) again based on the power meter reading. Allowing for the remaining uncertainty in the comparison of power at the two wavelengths, we conclude that within about 15% relative uncertainty the quantum yield is the same for doublet and quartet irradiation. This finding agrees with an earlier preliminary report and with parallel observations on Cr(NH3):+.l1 This photochemical data allow for a second, independent determination of vD. Since 68% of [Cr(en),13+ photothen with our photochemical yields chemistry is s~ow,~,' qD
=
4(Rxn)Qslow- 0.42 X 0.68 = o.64 $(RXn)D 0.45
This agrees well with the phosphorescence result and provides altogether a total of three independent, concordant measurements of vD at around 0.7. Since the earlier claims3i5of proof of doublet reactivity were based on v D = 0.3, they are clearly invalid. Before proceeding to a more detailed analysis we draw attention to the fundamental limitation that an absolute proof of doublet vs. quartet routes for the slow chemistry becomes more elusive as TD 1, and impossible on the basis of these types of kinetic considerations when vD = 1. In the ensuing discussion we will take, as round figures, t~ = 0.7, $(Rxn)g,hd = $(Rxn)Q,f,, +- $(Rxn)Qciow= 0.4, $(Rxn) f t = 0.13, $(RXn)D = 0.4. The %!erved value of qD still allows direct doublet reactivity for the slow reaction5 if 7]dr = 0.4 (vrisc = 0, v * = 0.40-0.13 for vpk = 0.7-0); i.e., coincidental equality of vh and $(Rxn)Q,hd. Apart from this required coincidence,
-
this modeling suffers from the deficiency, pointed out by Adamson et al.? that one would then anticipate a nonlinear Arrhenius plot for the phosphorescence yield and lifetime, contrary to ob~ervation.~J~-'~ Alternatively, the slow reaction could occur by reverse intersystem crossing and quartet reaction. This requires vriacto be close to unity? with q,, = 0.40-0.13 for the range vPke= 0.7-0. This model leads to acceptable values for the individual rate constants, despite contrary claims? and yields a linear Arrhenius plot for the phosphorescence with activation energy equal to or greater than the doublet/ quartet spacing (presently unknown). Between these extremes lies a continuum of fits involving both quartet and doublet reactivity. However, those which require major contributions of parallel routes for the slow chemistry again run into difficulties such as nonlinear Arrhenius plots. It has been observedg that doublet and quartet photoaquation lead to the same product distribution; this favors the reverse intersystem crossing, quartet reaction model. The route also minimizes the number of coincidences required and, on balance, is the most consistent with all of the evidence. Another complexity possible in this system should be noted. It may be that the prompt reaction occurs prior to the state Qo,Figure 1. In this event eq 1 could not be used to derive the parameter vQR, dowing a further degree of freedom in modeling the slow part of the photochemistry, but not altering the essence of the foregoing analysis. The division of TD into vpisc and qisc represents a final conundrum. The observation of fast rise times for phosphorescence suggests the prompt process, but arguments based on lifetimes and efficiencies support the slow process, unless kpk > Itisc. Any solution to this problem must await information about the Qostate, notably its lifetime, presently known only to be less than a few nanoseconds.
Acknowledgment. The authors thank the Natural Sciences and Engineering Council, the University of Victoria, and the Consiglio Nationale delle Ricerche of Italy for financial support. They also thank a referee for constructive criticism, some of which have been incorporated into this revised manuscript. (13) It has been pointed out that given the limited temperature ranges of some exisiting data, such a linearity criterion often allows some latitude in modeling the data with a two-component exponential.' However, it would seem difficult to do this for [ c r ( e ~ ~ ) ~ ] ~ + . ~ ' - ~ ~ (14) S. C. Chen, Ph.D. Thesis, University of Britich Columbia, 1970. (15) R. T.Walters and A. W. Adamson, Acta Chem. Scand.; Part A., 33, 53 (1979). (16)S. R.Allsop, A. Cox, T. J. Kemp, W. J. Reed, S. Sostero, and 0. Traverso, J. Chem. SOC.,Faraday Tram. 1, 76, 162 (1980).
