J. Phys. Chem. 1980, 84,891-898
Cr(NH3)4(SCN)F+and Cr(NHJ5F2+ is mainly phosphorescence.
Conclusion For those Cr(II1) complexes exhibiting relatively sharp emission, which therefore is phosphorescence, the emission quantum yield is roughly proportional to the lifetime. Thus the large range of quantum yields is attributable directly to processes degrading the 2E state rather than the variationis in intersystem crossing efficiencies. There seem to be little or no correlation between the separation of the absorption maxima of the quartet and doublet states and the quantum yield. In fact, there are pairs, for example, substitution of en by 2 ammines, in which large variations occur without change in spect,ral parameters. However, the ligands can be placed crudely in order of their effect on the quantum yield. Those complexes containing a t least one fluoro ligand tend to emit delayed fluorescence rather than phosphorescence Apparently, subtle changes in energy level and rate constants occur since the effect is not uniform.
Acknowledgment. This research was supported by grants from the Natural Sciences and Engineering Council Canada. References and Notes (1) (a) University of Victoria; (b) University of British Columbia. (2) Adamson, A. W.; Fleischauer, P. D. “Concepts of Inorganic Photochemistry”; Wiley: New York, 1975. (3) Parter, G. B.; Chen, S.N.; Schlfer, H. L.; Gausmann, H. Jheor. Chim. Acta 1971, 20,81. (4) Castelli, F.; Forster, L. S. J . Am. Chem. SOC. 1973, 95, 7223. (5) Yardley, J. T.; Beattie, J. K. J. Am. Chem. SOC.1972, 94, 8925. (6) Chen, S.N.; Porter, G. B. Chem. Phys. Left. 1970, 6, 41. (7) Pyke, S.C:.; Windsor, M. W. J. Am. Chem. SOC.1978, 700, 6518. (8) Ohashi, Y.; Kobayashi, T. J . Phys. Chem. 1979, 83, 551. (9) Kirk, A. D.; Hoggard, P. E.; Porter, G. B.; Rockley, M. G.; Windsor, M W. Chem. Phys. Lett. 1976, 37, 199. (10) Chatterjee, K. K.; Forster, L. S. Spectrocbim. Acta 1964, 20, 1603.
89 1
(11) Kane-Maguire, N. A. P.; Langford, C. H. J . Chem. Soc., Cbem. Commun. 1971, 895. (12)Chen, S. N.; Porter, G. B. J. Am. Chem. SOC. 1970, 92, 3196. (13) Pfeil, A. J . Chem. SOC. 1971, 93, 5395. (14) Targos, W.; Forster, L. S. J. Chem. Phys. 1966, 44,4342. (15) Demas, J. N.; Crosby, G. A. J . Chem. SOC. 1970, 92, 72152. (16) Zander, H. -U. Doctoral thesis, Johann Wolfgang Goethe University, Frankfurt. - -., 1969. . (17) Sandrini, D.; Gandolfi, M. T.; Moggi, L.; Balzani, V. J . Am. (%em. SOC. 1978, 700, 1463. (18)Walters. R. T.: Adamson, A. W. Acta Cbem. Scand., Ser. A 1979, 33. 53. (19) W e , S.C.; Ogasawara, M.; Kevan, L.; Endicott, J. F. J. Phys. Chem. 1978, 82,302. (20) Conti. C.: Forster, L. S. J. Chem. SOC. 1977, 99, 613. (21) Bifano, C.;Linck, R. G. Inorg. Chem. 1974, 73, 609. (22) Kirk, A. D.; Kelley, T. L. Inorg. Chem. 1974, 73, 1613. (23) Manfrin, M. F.; Sandrini; D., Juris, A.; Gandolfi, M. T. Inorg. Chem. 1978, 77,90. (24) Kirk. A. D.: Wona. C. F. C. Inora. Chem. 1979. 78. 593. (25) Ballardini, k.; VaFani, G.; Wasgestian, H. F.; Moggi, L.; Balzcmi, V. J . Phys. Chem. 1973, 77, 2947. (26) Langford, C. H.; Tipping, L. Can J. Chem. 1972, 50, 887. (271 Wona. C. F. C.; Kirk, A. D. Inora. Chem. 1977, 76.3148. (28) Kirk,-A. D.; Frederick, L. A.; W&g, C. F. C. Inorg. Chem. 1979, 78, 448. (29) Zinato, E.; Lindholm R. D.; Adamson, A. W. J . Am. Chem. SOC. 1989, 97, 1076. (30) Adamson, A. W.; Wegner, E. E. J. Am. Cbem. SOC.1966, 88,394. (31) Kirk, A. D.; Porter, G. B. Inorg. Chem., in press. (32) Wong, C. F. C.; Kirk, A. D. Inorg. Chem. 1976, 75, 1519. (33) Wright, R. E.; Adamson, A. W. Inorg. Chem. 1977, 76, 3630. (34) Chiang, A.; Adamson, A. W. J . Phys. Chem. 1968, 72,3827. (35) Porter, G. B.; Schlafer, H. L. Z. Phys. Cbem. (FrankfurtamMain) 1964, 40,280. (36) Camassei, F. 11.; Forster, L. S. J. Cbem. Pbys. 1969, 50, 2603. (37) Bolletta, F.; Maestri, M.; Balzani, V. J . Phys. Cbem. 1976, 80, 2499. (38) Forster, L. S.Adv. Chem. Ser. 1976. No. 750, 172. (39) Maestri, M.; Bolletta, F.; Moggi, L.; Balzani, V.; Henry, M.; Hoffman, M. Z. J. Am. Chem. SOC. 1978, 100, 2694. (40)The doublet state lies 13.8 X lo3 cm” above the ground state; thus the lowest quartet would be 8 kcal N 2.8 X lo3 cm-‘ higher. The Stokes shift between the absorption maximum and the (unobservable) fluorescence maximum is approximated by assuming the quartet lowest level to be halfway between these maxima. (41) Schlafer, H. L.; Gausmann, H.; Zander, H. -U. Inorg. Chem. 1967, 6 , 1528. (42) Koglin, E.; Krasser, W. 2.Naturforsch. A 1973, 28, 1131.
Proton-Induced Fluorescence Quenching of 2-Naphthol Chris M. Harris and Ben K. Selinger” Chemistry Department, Australian National University, Canberra, A.C.T. 2600, Australia (Received July 2, 1979)
The combination of steady-state and time-dependent measurements allows the estimation of the rate constants for excited state acid-base reactions, including the rate constants for the diabatic quenching of the excited molecules by the added acid. These rate constants are significant for the 2-naphthol-2-naphtholatesystem [(2.4 f 0.3) X lo7 s-l M-l; (6 1)X lo9 s-l M-l] and their neglect in the published results on this system has led to errors in the rate constants for deprotonation and reprotonation. The most significant result is the pronounced departure at low pH from an adiabatic transfer of electronic energy.
*
1. Introduction The dissociation of 2-naphthol in the excited state was investigated first by Forsterl and later by Weller.2-4 Fluorescence from both ROH* and RO-* in the pH range pK (9.3) to pK* (2.8) was observed by Forster. His explanation was that the short excited state lifetime allowed insufficient time for complete dissociation. At low pH it was shown tlhat the deprotonation in the excited state became reversible. 0022-3654/80/2084-089 1$0 1 .OO/O
The rate constants for 2-naphthol have been calculated from data obtained by using either time-dependent studies alone6-I or a combination of steady-state and time-dependent m e t h o d ~ . ~ - ~ Both , ~ - l ~techniques are generally thought to be of equal merit, though it has been suggested6 that more information about the reacting system cam be obtained by analysis of time-dependent results. A decrease in the fluorescence of ROH* at low pH was initially attributed by Weller2 to quenching by the anions 0 1980 American Chemical Society
The Journal of Physical Chemistry, Val. 84, No. 8, 1980
892
Harris and Selinger
present. He later4 stated that quenching by H+ at low pH was taken into account, where necessary, in his analysis of the experimental data. The studies of 2-naphthol which have been reported since then have not allowed for the possibility of a proton-induced quenching mechanism, in spite of the reports of significant diabatic quenching'l by protons of 1-and 2-ethylnaphthoate,12 1- and 2-naphthoic acid,13 1- and 2-naphthamide,14 methyl 1-15 and methyl 2-naphthyl ketone,16 1- and 2-naphthylamine,17 and 1naphthol.47 In order to clarify the fluorometric pH titration behavior of 2-naphthol and to investigate whether time-dependent methods lead to more information about the reaction system, we have measured fluorescence lifetimes and relative fluorescence quantum yields of 2-naphthol at various proton concentrations. These results were then processed in the following way to yield the rate constants in the excited state. (a) Steady-State Method. The equations relating the relative quantum yield of fluorescence to pH for the deprotonation of an acid in the excited state have been derived by Weller in his analysis of the deprotonation of 1and 2-naphth01.~~~ The present analysis allows h and h', the quenching constants for the quenching of ROH* and RO-* by the added hydrochloric acid, to be determined independently of the other rate constants. At pH values substantially below the ground state pK the naphthol is undissociated and unprotonated in the ground state. Proton transfer in the excited state occurs according to the following scheme, in which ROH is excited by absorption of light and then undergoes the reactions shown:
where hf and k j are rate constants for emission of fluorescence, h, and h,' describe radiationless decay of the excited state, h and h'refer to the quenching action of the added acid, and hl and h i are the forward and reverse rate constants for the deprotonation reaction. Throughout this paper umprimed quantities refer to ROH*, primed quantities to RO-*. The relative quantum yields of fluorescence from ROH* and RO-* are given by
6 -- 1 + (h2' _
+ h?70'(?+)2[H+1 D
60
6'
h170
60
D
-=-
(2)
which can be obtained by application of steady-state kinetics2 to reaction scheme 1. $/@o represents the relative fluorescence quantum yield for ROH*, $'/$o/ the relative fluorescence quantum yield for RO-*, and T~ and TO/ are defined by 1 -=hk,+k, - -- k( + h[ (4) 70
701
The screening effect of the ionic atmosphere on the charged reagents (RO-* and H+) is allowed for plotting [H+](yJzzl instead of [H+],where z1 is the charge on the base and y+.is the mean activity coefficient of the solution. It has been shown'* that this correction is applicable to
proton transfer in the excited state and then results in rate constants which are independent of ionic strength. The assumption here is that the quenching of RQ-* is by H+ rather than by the anion. The results (section 3) suggest that such is the case. Equations 2 and 3 lead to the following expression for the ratio of the relative quantum yields:
When the ratio of the experimentally determined quantum yields is plotted against (T+)~[H+] the points are expected to give a linear plot of slope ( h i + h ? ~ , ' / ( h ~ ~ ~ ) and intercept (h170)-'. If T~ and are known, and k i >> h' then ( 5 ) leads to the determination of k l / h i , the equilibrium constant in the excited state, and hence to pK*. At low pH h
h'
If the diabatic quenching is negligible, this expression equals unity, otherwise (6) and (2) may be rearranged to give eq 7-9.
obtained from With the values of h170and ( h i + k ( 5 ) ,the left-hand sides of (7)-(9) may be plotted against (h2'+ h?7,'[Hf] to give linear plots of slope h~~ and intercept h'7,'k170 + k ~ ~ / ( ? + )It~ should . be noted that (7) and (8) are generally more useful than (9), because at low pH the experimental values of 6/qh0and 6/6,, + 6/14,,' are more reliable than the values of 6'/$o/, because these are approaching zero. Supplementary measurements required are the determination of TO/ and T~ At pH values greater than pK the fluorescence is of RO-* and RQ- is also the form which absorbs the extinction light. The decay should follow a single exponential with lifetime 7,'. A t neutral pH, well below pK, only ROH will be present in the ground state. Proton transfer in the excited state will be determined by kl and 1 / (the ~ competing ~ processes at neutral pH for the deactivation of ROH*) and will be irreversible. ROH* should decay as a single exponential with lifetime T. 7 0 is calculable from 70 = 7 / ( 6 / 6 0 ) b H 7) (10) The equations relating the quantum yields of fluorescence to pH for the protonation of a weak base in the excited state have been derived by Kokubunlg and Wat-
Fluorescence Quenching of 2-Naphthol
The Journal of Physical Chemistry, Vol. 84, No. 8, 1980 893
kins.la The reactions corresponding to the scheme for a weak acid are as follows:
The full analysis is given below.20 ( b ) Time-Dependent Method. An analysis of the exponential character of fluorescence decay curves at different pH has yielded estimates for the rate constants in the excited state.5 The amounts of decay of the ROH* and RO-* species are expressed by [ROH*] =
transmitted a significant fraction (5-6%) of the ROH* emission. It is important to allow for such overlap of fluorescence emissions when analyzing both steady-state and kinetic results, so that experimental artifacts do not result in an erroneous analysis. At pH values well below pK* any RO-* formed from ROH* is quickly reprotonated or deactivated and ROH* is found to decay as a single e ~ p o n e n t i a l : ~ ~ [ROH*]
exp[-Alt]
(20)
where A1
= ((a + u’) - ( ( a-
+ 4klhz’(r*)2[H’]}1’2},/2
We have from section 3 that a t low pH, (a - ~ ’ >> 1 ~ 4klk,’(y,)2[H+] imd the square root term can be expanded as follows: A1
= (a
+ a’) - (a’-
a ) ( l + 2hlh,l(y*)2[H+]/(~ - a?’) _- 2 1/70 kl k[H+] klhz’(r*)2[H1 1/70/ - 1/70 - hl + (hz’ + h ’ - h/(r*)2)(r*)z[H+]
+ +
where X1,Z
= ((u+ a’)
=F ( ( a -
u=1 u ’ = 1/7d
+ 4h1h2’(r*)2[H’])1’2)/2 (14) / +~kl~ + k[H+] (15)
+ (h2’ + h?(r.JfH+]
UU’-
hlhi(~+)~[H+]
- 1/70 - hl
