5070
J . Phys. Chem. 1993,97, 5070-5073
Quenching Mechanism in a Highly Exothermic Region of the Rehm-Weller Relationship for Electron-Transfer Fluorescence Quenching Koichi Kikuchi,’ Taeko Niwa, Yasutake Takahashi, Hiroshi Ikeda, and Tsutomu Miyashi Department of Chemistry, Faculty of Science, Tohoku University, Aoba, Aramaki, Aoba- ku, Sendai 980, Japan Received: February 16, 1993
The fluorescencequenching mechanism in a highly exothermic region of the Rehm-Weller relationship is shown to be a long-distance electron transfer for producing the geminate radical ion pair with fluorescer radical cation in an electronically excited state and quencher radical anion in the ground state.
1. introduction In previous work,14 we have revealed that the fluorescence quenching mechanism of an electron donor-acceptor (EDA) system depends on (i) the effective quenching distance y a and (ii) the free enthalpy change AGr (= Elpox- E ( & ) ) of actual electron transfer (ET) in addition to (iii) the solvent polarity. Here E1l2OX and E I l Z r are 4 the oxidation potential of the electron donor and the reduction potential of the electron acceptor, and E ( S I )is the excitation energy of the fluorescent state. On the basis of such a new aspect, we have critically reconsidered the Rehm-Weller (RW) relationship between the fluorescence quenching rate constant k, and AGr, and we proposed a criterion for determining the quenching mechanism in a highly exothermic (or the inverted) region (AGr < -2.0 eV).4v5 The criterion is based on both measurement of y a and the free radical yield @R of one encounter fluorescence quenching: When y a is much greater than 3 A, the quenching is due to a long-distance ET (Le., ET not at contact distances), and the geminate radical pair (GRP) as the primary quenching product is the ground-state GRP or the excited-state GRP, the counterpart of which is electronically excited. If @R is well related to the free enthalpy change AGb (= EIIZred - E l p o x )of back ET within the ground-state GRP by a semiclassical theory,6 the primary quenching product is the ground-state GRP. If @R is not explained in terms of AGI,, the primary quenching product may be the excited-state GRP alone or the miture of the excited-state GRP and the ground-state GRP. In order to determine the quenching mechanism in a highly exothermic region, the criterion has first been applied for the EDA systems consisting of anthracenecarbonitriles (AC) as fluorescers and aromatic amines as q~enchers.~It has been established that the quenching in the region AGf < -2.0 eV is due to a long-distance ET for producing the geminate radical ion pair with the fluorescer radical anion in an electronicallyexcited state and the quencher radical cation in the ground statea7 In this work we determine the quenching mechanism for the EDA systems actually involved in a highly exothermic region (AGf < -2.0 eV) of the RW relationship, because the lack of inverted region in the RW relationship has been one of the most important problems in photochemistry since 1970. Recently, Mataga et aL8 studied these systems by a picosecond laser photolysis method, but they have not succeeded in obtaining any information on the quenching mechanism other than a longdistance ET for producing the ground-state GRP.
2. Experimental Section Perylene (Per; GR grade, TokyoKasei) was recrystallized from toluene three times. 1,lZBenzoperylene (BPer; Aldrich) was purified by column chromatography on silica gel. 9,lO-Diphenylanthracene (DPA; SP grade, Nakarai) was used as received.
Pyrene (Py; CP grade, Tokyo Kasei) was zone refined. Fluoranthene (Fl; GR grade, Nakarai) was purified by column chromatography on alumina, recrystallized from ethanol, and sublimatedunder vacuum. 9,lO-Dicyanoanthracene(DCA), 2,9,10-tricyanoanthracene (TrCA), and 2,6,9,10-tetracyanoanthracene (TeCA) are the same compounds used in previous work.I-4~~ Tetracyanoethylene ( T C N E Aldrich) and fumaronitrile (DCNE; GC grade, Tokyo Kasei) were sublimated three times under either argon or nitrogen atmosphere just before experiments. Acetonitrile (SP grade, Nakarai) was distilled repeatedly on calcium hydrideunder argon atmospherejust before experiments. Absorption spectra were recorded on a Hitachi 330 spectrophotometer. Fluorescence spectra and fluorescence excitation spectra were measured by a spectrophotometer built in this laboratory. Fluorescence lifetimes were measured by a timecorrelated single photon counting method. Transient absorption spectra were measured by a conventional flash photolysis. Both @Randthe triplet yield @T of one encounter fluorescencequenching were determined by an emission-absorption flash photolysis method.9 This method measures the transient absorption and the time-integrated fluorescence intensity during a flash simultaneously. The former is used to know the initial concentration of free radicals, and the latter is used to evaluate the amount of light absorbed by a sample solution. Elpox and versus SCE were measured in acetonitrile solution containing 0.1 M tetraethylammonium perchlorate as supporting electrolyte. Deaerated solutions were used to measure the transient absorption spectra and to determine the free radical yields. Aerated solutions were used to determine the effective quenching distance. All measurements were made at 298 K.
3. Results and Discussion Absorption spectra indicated that the EDAcomplex formation between the fluorescers and the quenchers (TCNE and DCNE) is negligible in the ground state up to the quencher concentration of 0.1 M. Fluorescence was quenched by the addition of either TCNE or DCNE, but the fluorescence spectral shape was not changed. E(SI)’s were determined from the absorption and fluorescence spectra as listed in Table I. Fluorescencequenchingrateconstants k, were determinedfrom the Stern-Volmer plot at the quencher concentration below 10 mM. They are close to the diffusion-controlled limit as shown in Table I1 except for the F1-DCNE system. To determine ya, we have measured the fluorescence yield up to the quencher concentration of 0.1 M. The values for ya were evaluated using a modified Stern-Volmer equation derived by Weller,lo and they were listed in Table 11. The ya for the FI-DCNE system is 3 A, which is just the interplanar separation in the excimer and charge transfer complex.
0022-365419312097-5070$04.00/0 0 1993 American Chemical Society
Electron-Transfer Fluorescence Quenching
The Journal of Physical Chemistry, Vol. 97, No. 19, 1993 5071
TABLE I: Oxidation Potentials versus SCE, Excitation Energies of Fluorescent States, E(&); Fluorescence Lifetimes in Deaerated and Aerated Solutions, T ; Molar Extinction Coefficients of Radical Cations, CR(X), and of T-T Absorption, *(A) 7 (ns)
fluorescer TeCA TrCA DCA F1 PY DPA BPer Per
Elpox(V vs SCE)
2.20 2.01 1.89 1.65 1.24 1.19 1.03 0.98
E(&) (eV) 2.89 2.89 2.89 3.04 3.36 3.09 3.10 2.85
deaerated
aerated
17.8 17.5 15.3 45 310 8.7 118 5.5
14.7 14.8 12.7 27 14.9 5.8 14.9 4.2
(h4-1cm-I)
e&+)
cr(X)
(M-’ cm-I)
9.3 X 103 (395 nm) 3.2 X lo4 a (412nm) 1.1 X lo4 (450nm) 3.6 X 104 (460nm) 1.4 X lo4 (482nm)
1.14X lo4 a (445 nm) 7.4 X 103 (590nm) 3.96 X lo4 (515 nm) 3.5 x io4 (535 nm)
Reference 17. Reference 16.
TABLE Ik Free Enthalpy Changes of Photoinduced ET, A Q Fluorescence Quenching Rate C ~ ~ t a n t s , Free Enthalpy Changes of Back ET within the Ground-State GRP, ACa; Effective Quenching Distance, ya; Free Radieal ield, @R; Triplet Yield, @T; Rate Constants of Spin-Allowed Back ET within GRP Determined Directly by Picosecond Laser Photolysis86 and Indirectly from @R by Use of Eq 1, kb
%
Quencher: DCNE
fluorescer
Fl Per DPA BPer PY
-AGf (eV) 0.03 0.51 0.54 0.71 0.76
kq (loioM-1
s-I)
0.061 1.9 1.4 1.8 1.8
70 (4
-AGb (eV) 3.01 2.34 2.55 2.39 2.60
3.0 4.5 6.5 6.5 4.9
@R
@T
0.019 0.062 0.035 0.036
0.35 0.010 0.033 0.015 0.020
kb (s-I) (indirect) 8.2 X 1O1O 2.3 X loio 4.3 x 1010 4.2X 10’0
Quencher: TCNE
kb (9-l) fluorescer TeCA TrCA DCA
F1 Per DPA BPer PY Reference 8b.
-AGf (eV) 0.91 1.07 1.22 1.61 2.09 2.12 2.29 2.34
k, ( 1Olo M-I 2.0 2.4 2.6 3.0 2.7 2.4 2.5 2.3
SKI)
-b& (ev)
70 (‘9
1.98 1.82 1.67 1.43 0.76 0.97 0.81 1.02
5.2 5.5 8.2 9.4 7.9 8.7 11 11
@R
0.012 0.013 0.016 0.074 0.14 0.047 0.061 0.033
direct‘
6.1 X lo8 1.3 x 109 1.9 x 109 2.6 x 109
indirect
1.3 X 1.2x 9.8 X 2.0 x 9.8 x 3.2 X 2.5 X 4.7 x
10” 10” 1O’O 10’0 109 1Olo 1Olo 10’0
In contrast, the y a for the other EDA systems are greater than
3 A. Therefore, the fluorescence quenching of the FI-DCNE system is induced by exciplex formation. But the quenching of the other systems is induced by long-distance ET. The AGrvalues are calculated as listed in Table 11. Then we used the E(Sl)’s and the E1poX’s listed in Table I and the E1prCd’s(versus SCE) of +0.22 V for TCNE and -1.36 V for DCNE. The quenching of the FI-DCNE system by exciplex formation is consistent with our previous proposition’-5 that the ET fluorescence quenching is induced by exciplex formation when AGf > -0.4 eV. Most recently, Tachiya and Muratall clearly demonstrated within the framework of the Marcus theory12 that the favorite quenching distance increases with decrease in AGf. This agrees with the fact that ya increases with decrease in AGf. Flashing of the acetonitrile solution containing TCNE and one of Per, BPer, DPA, and Py gives the transient absorption spectrum, which is the superposition of the absorption spectra of hydrocarbon radical cation and TCNE radical anions, TCNP-. Figure 1 shows the transient absorption spectrum obtained for the deaerated solution containing 10 pM Per and 1 mM TCNE. The absorption maxima at 18 700 cm-I (535 nm) and 13 300 cm-I (750nm) areattributed to thePerradicalcation.I3 Flashing of the solution containing TCNE and one of Fl, DCA, TrCA, and TeCA gives the transient absorption spectrum shown in Figure 2, which is assigned to the spectrum of TCNP-.I3 The radical cations of F1, DCA, TrCA, and TeCA are considered to be too unstable in acetonitrile to detect by a conventional flash photolysis method. Flashing of the solution containing DCNE and one of Per, DPA, BPer, and Py gives the transient absorption spectrum which is the superposition of the triplet-triplet (T-T) absorption of hydrocarbon and the absorption due to hydrocarbon radical
10000
W A V E N U MBE R / c m - ’
25000
Figure 1. Transient absorption spectra obtained for the deaerated acetonitrile solution containing 10 WMPer and 1 mM TCNE.
cation. There is no absorption due to DCNP- in the wavelength region greater than 450 nm.I3 Flashing of thesolution containing F1 and DCNE gives only the T-T absorption of F1. The triplet energies E(TI) for Per, DPA, BPer, and Py have been reported to be 1.56, 1.77,2.01, and 2.08 eV, respectively.14 Comparing E(TI) with lAGd, we find that the triplet-state ET from these hydrocarbonsto the quencher is energeticallypossible when TCNE is used as the quencher, but not when DCNE is used as the queneher. i p and ~ @T were determined at the quencher concentration below 10 mM by use of the emission-absorption flash photolysis? and they are listed in Table 11. Then, a deaerated ethanol solution containing 0.1 mM 9-phenylanthracene was used as the actinometer solution. When Per, BPer, DPA, or Py was used as the
Kikuchi et al.
5072 The Journal of Physical Chemistry, Vol. 97, No. 19, 1993
0.02w 0
a
U
m U 0
A
rn
\i
/
m U
I " "
0 10000
W A V E N U MBE R/c m - '
25000
8
Figure 2. Transient absorption spectra obtained for the deaerated acetonitrile solution containing 50 jtM F1 and 2 mM TCNE.
fluorescer and TCNE was used as the quencher, the initial absorbance was obtained by monitoring at the absorption maximum of hydrocarbon radical cation. When F1, DCA, TrCA, or TeCA was used as the fluorescer and TCNE was used as the quencher, the initial absorbance was obtained by monitoring at the absorption maximum of TCNE*-, 445 nm. The molar extinction coefficient for T C N E - has been reported to be 6520 M-1 cm-l at 445 nm.I5 Themolar extinctioncoefficientsof radical cations for Per'(' and PyI7 are reported in Table I. The molar extinction coefficients for radical cations of BPer and DPA were determined by use of the photoinduced ET from these hydrocarbons to TeCA as listed in Table I. The molar extinction coefficient for the TeCA radical anion was already determined to be 7500 M-I ~ m - l .When ~ Per, DPA, BPer, or Py was used as the fluorescer and DCNE was used as the quencher, both initial absorbanceof the radical cation and the hydrocarbon triplet were determined by monitoring at their absorption maxima. The molar extinction coefficients tr(X) of the T-T absorption for Py and Per have been reported as listed in Table I.I6J7 The ET'S for F1, DPA, and BPer were determined by triplet energy transfer as listed in Table I. The large value of @T for the Fl-DCNE system is more evidence for the quenching due to exciplex formation. If the fluorescencequenching is induced by a long-distance ET for producing the ground-state GRP, @R is described by eq 1 when E(T1) > IAGbl.
% = kesc/(kesc
(1) When E(TI) IIAGbl, the hydrocarbon triplet may be produced by the geminate radical recombination. Then @R and @T are given by eqs 2 and 3. @R @T
+ kb)
= k e s c / ( k e s c + kb + ' k c )
(2)
= k,sc(kesc + kb +
(3)
Here k,,, is the rate constant for the geminate radical separation into the free radicals, and kb and kist are the rate constants for the spin-allowed and the spin-forbidden back ET within the ground-state GRP, respectively. The values for keSchave been reported by Mataga et alagb for the following EDA systems: 0.8 X 109 s-I for Per-TCNE, 1.9 X 109 s-I for BPer-TCNE, 1.7 X 109 s-I for DPA-TCNE, and 2.5 X lo9 s-I for Py-TCNE. The diffusion coefficient in acetonitrile does not so much depend on the molecular size:18.19 e.g., 2.74 X 10-5,2.55 X 10-5,and 2.35 X cmz S-I for naphthalene, anthracene, and perylene, respectively. In this study, therefore, we assume k,,, to be the average of the above rate constants, 1.6X lo9 s-I. Then the kb values are calculated from @R by use of eq 1 or from @R and @T by use of eqs 2 and 3. They are listed in Table 11. The kb values reported by Mataga et a1.8b are also listed in Table 11. We find
0
1
2
3
-AGb / eV Figure 3. Plot of kb versus AGb: ( 0 )determined directly from the decay curve of GRP by Mataga et a1.;8b(0,A) determined indirectly from *R by use of eq 1 .
that the kb values reported by Mataga et al. are 1 order of magnitude smaller than those determined from @R. Figure 3 shows the plot of kb versus AGb. A typical bellshaped free enthalpy dependence of kb is observed for the kb(s determined from @R (and @T) of the EDA systems with AGb < -1.4 eV (open circles) and the k{s reported by Mataga et al. (solid circles). The bell-shaped curve in this figure was depicted according to eq 4 with the following fitting parameters:
where S = Xv/hv, the electronic coupling matrix element V = 28 cm-I, the solvent reorganization energy XS = 1.65 eV, the reactant vibrational reorganization energy XV = 0.3 eV, and the average energy of active vibrational mode hv = 1500 cm-I. The k{s indicated by open and solid circles are just on the theoretical curve. In contrast, the kb(s (triangles) for the systems of PerTCNE, BPer-TCNE, DPA-TCNE, and Py-TCNE, which were determined from @R by use of eq 1, deviate upward from the theoretical curve. One may find that the above fitting parameters are reasonable, if one may compare them with the fitting parameters reported for the other EDA~ystems.3~~~20 y a is greater than 3 A for every case. According to the criterion described in the Introduction, therefore, the quenching mechanism is the longdistance ET for producing the ground-state GRP in the case of the EDA systems with AGf > -2.0 eV, but it is the long-distance ET for producing the excited-state GRP in the case of the EDA systems with AGf < -2.0 eV. As suggested in previous work5s7 and as predicted by the theoreticalstudy by Tachiya and Murata,I I the switchoverof the quenching mechanism in a highly exothermic region occurs at AGf = -2.0 eV. The absorption spectra of Per*+,BPer*+,DPA*+,and Py*+are located in the near-infrared region,13but the absorption spectrum of T C N P - is located in the blue region (see Figure 2). Therefore, the excited-state GRP consists of the fluorescer radical cation in the electronically excited state and TCNE*-in the ground state. The free enthalpy changes AGr* of ET fluorescence quenching for producing such excited-state GRP are calculated as foflows: -0.44, -0.77, -0.53, and -0.79 eV for Per, BPer, DPA, and Py, respectively. Then we used 1.65,1.51, 1.59,and 1.55 eV as the electronicexcitation energies for Per'+, BPer'+, DPA*+,and Py*+, respectively. AH the A%f* values are more negative than -0.4 eV, so that such excited-state GRP may be generated by a long-
Electron-Transfer Fluorescence Quenching distance ET with the quenching rate constant of diffusioncontrolled limit. Indeed, the y a values for these EDA systems are much greater than 3 A. Therefore, it is concluded that in a highly exothermicregion of the RW relationshipthe fluorescence quenching is induced by the long-distance ET for producing the excited-state GRP at the diffusion-controlled limit, even if the fluorescence quenching due to the long-distance ET for producing the ground-state GRP is not so fast as to give the diffusioncontrolled limit. This may be the reason why there appears no inverted region in the RW relationship. The energy gaps IAGb*l between the excited-state GRP and the ground-state neutral molecules are 2.41,2.37,2.56,and 2.57 eV for the systems of Per-TCNE, BPer-TCNE, DPA-TCNE, and Py-TCNE, respectively. In the case of these systems the total reorganization energy A,,,I (= AS + A,) = 1.95 eV is much closer to IAGb*I than to IAGbl. Then the geminate radical recombination into the ground-state neutral molecules occurs more rapidly within the excited-state GRP than within the groundstate GRP. Therefore, @R is expected to be smaller for theexcitedstate GRP than for the ground-state GRP. This is the reason why the kb obtained indirectly from @R by use of eq 1 is much greater than the kb determined directly from the decay analysis of the ground-state GRP.
Acknowledgment. The authors aregreatly indebted to Professor H. Kokubun for his interest in this work and for generous support. References and Notes (1) Kikuchi, K.; Niwa, T.; Takahashi, Y.; Ikeda, H.; Miyashi, T.; Hoshi, M. Chem. Phys. Lett. 1990, 173,421424.
The Journal of Physical Chemistry, Vol. 97, No. 19, I993 5073 (2) Kikuchi, K.; Hoshi, M.; Niwa, T.; Takahashi, Y.; Miyashi, T. J . Phvs. Chem. 1991. 95. 3 9 4 2 . (3) Kikuchi, K.;Takahashi,Y.; Hoshi, M.;Niwa,T.;Katagiri,T.; Miyashi, T. J. Phys. Chem. 1991, 95, 2378-2381. (4) Kikuchi, K. J . Photochem. Photobiol. A: Chem. 1992.65.149-156. (5)
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