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Influence of temperature on fluorescence quenching of 9,10-dicyanoanthracene by durene in acetonitrile
JOURNAL
OF
LUMINESCENCE ELSEVIER
Journal
of Luminescence
65 (1996) M-348
Influence of temperature on fluorescence quenching of 9, lo-dicyanoanthracene by durene in acetonitrile Marek Mac*, Piotr Kwiatkowski, Andrzej M. Turek Received
31 August
1995: accepted
13 September
1995
Abstract The effect of temperature on the fluorescence quenching of 9,10-dicyanoanthracene has been investigated. The fluorescence quenching in the system is accompanied
(DCA) by durene in acetonitrile by a weak exciplex emission. The
exciplex fluorescence quantum yields and exciplex lifetimes depend strongly on temperature and the exciplex emission is efficiently quenched by lithium perchlorate. The most important pathway of deactivation of the contact ionic pair (GRIP) is salvation of the CRIP. The structure of the CRIP has been discussed on the basis of the analysis of the exciplex fluorescence spectra as well as the temperature dependence of the fluorescence quenching rate constants.
1. Introduction Fluorescence quenching of 9,10-dicyanoanthracene (DCA) by organic neutral donors in acetonitrile has been intensively investigated [l-4]. The presence of the radical anions of DCA and the radical cations of the donors forming in the primary electron transfer step was observed in the transient absorption spectra or in the photocurrent experiments. Usually, only free radicals are the transient intermediates observed in the nanosecond or microsecond flash photolysis experiments. The magnitude of the charge separation quantum yields depends on the back electron transfer rate constants. There are some reports on the formation of the emissive species of DCA and several organic
*Co)-responding
author
0022-2313~96~$15.00 ( 1996 SD/ 0022-23 13(95)00079-8
donors in acetonitrile [5,6]. Such species arc called exciplexes (excited complexes). Usually. the exciplexes were observed in the appropriate donor sacceptor systems in weakly polar solvents. Recent papers of Gould et al. [6] showed the exciplex emission in the systems containing strong electron acceptors such as 9.1 O-dicyanoanthracene (DCA) or 2,6,9.10-tetracyanoanthracene (TCA) and electron donors (alkyl derivatives of benzene) in acetonitrile. The mechanistic scheme that describes the processes following the electron transfer fluorescence quenching has been presented [6]. This is shown in Fig. 1. Electron transfer fluorescence quenching leads to the solvent separated radical ion pair (SSRIP = A-(S)D+) or to the contact radical ion pair (CRIP = A-D+). The two radical species differ from each other in electronic coupling, which is much higher in the latter case. This factor is responsible for the fluorescence resulting from the CRIP.
Elsevier Science B.V. All rights reserved
342
M. Mac et al. J Journal
c-
A*+D
of Luminescence 65
7
hv hv . -
4kCetlcp AD
AWD
Fig. 1. Photophysical processes following fluorophore in the presence of the quencher solvents.
A+Dl
the excitation of molecules in polar
Excitation of the charge transfer complex (AD) leads directly to the CRIP. A higher efficiency of the emission from the CT state observed after excitation of the AD complexes compared to that of the CT state produced by diffusive quenching of A* by D indicates the importance of the production of SSRIP without the exciplex intermediate. Both species (SSRIP and CRIP) may dissociate into free radicals (A- + D+) which are usually observed in the nanosecond and microsecond transient absorption experiments. It is difficult to differentiate between the CRIP and SSRIP states on the basis of the transient absorption measurements due to similarity of their absorption spectra. In general, the exciplex is a mixture of the states (A-D+ Z$ A*D). If the admixture of the locally excited state A*D is negligible, the term exciplex is equivalent to the CRIP state. It is assumed that in polar solvents such as acetonitrile the mixing of other states than CT is negligible [6]. Thus, in such systems the shape of the emission envelope may be analyzed in terms of Marcus theory [7]. Such a type of analysis provides the values of the important parameters characterizing the electron transfer process, i.e., the free energy of the back electron transfer process (AG) and the reorganization energies (ni, and j,) [6,8] as presented in papers of Gould et al. [6]. It has been found that the fluorescence quantum yields of the excited DA complexes of TCA with methyl derivatives of benzene are very low due to very fast other deactivation processes such as nonradiative back electron transfer process (k, _ +,,) and solvation (k,,,,). The fluorescence quantum yields of the DCA exciplexes, formed in the colli-
11996) 341-348
sional way from the encounter complex (A*(S)D) are much higher due to the larger reduction potential of DCA as compared with TCA (Ered @CA) = - 0.91 eV/SCE, Ered(TCA) = - 0.44 eV/SCE) which makes the AG more negative in the case of DCA exciplexes. Thus, the nonradiative back electron transfer in CRIP is slower in the DCA complexes (inverted Marcus region). The aim of this work is to analyze the temperature behavior of the exciplexes of DCA and durene in acetonitrile using steady-state and pulse fluorescence techniques.
2. Experimental DCA (Tokyo Kasei) was crystallized from toluene. Durene (Koch-light Laboratories) was sublimed under vacuum. Acetonitrile of spectroscopic grade (Fluka) was used without purification. The absorption spectra of the mixture of DCA and durene were strictly additive indicating no ground state complexation in the quencher concentration used in experiments (up to 0.145 M). The steady-state measurements were performed using a conventional spectrolluorimeter with a cooled photomultiplier EMI 955 8B operating in a single photon counting mode. For excitation the 405 nm Hg line was selected. The sample was held in a high and low temperature cryostat (Oxford Instruments, DN1704). The fluorescence spectra were corrected for the spectral sensitivity of the detecting system. The fluorescence decay functions were recorded using the time-correlated single photon counting arrangement equipped with a Hamamatsu photomultiplier (R3809U MCP-PMT), with a nitrogen flash lamp as an excitation source (half-width ca. 1.2 ns) [9]. For excitation the 405 nm line was selected. The fluorescence lifetimes of the mixtures of DCA and durene were calculated by fitting the experimentally recorded fluorescence decay functions to the convolution of a monoexponential decay function and excitation profile. Stern-Volmer plots were constructed from the lifetimes of the perturbed DCA samples. The SternVolmer plots were linear within the quencher concentration
range used in the experiments. The changes of the concentration of the perturber due to the changes of temperature were taken into account in the calculations of SternPVolmer rate constants. The decays of the exciplex fluorescence were measured at 580 nm. The exciplex fluorescence spectra of DCA and durene in acetonitrile were extracted from the emission spectra after subtracting from them the residual monomer fluorescence multiplied by an appropriate factor. For the estimation of the exciplex fluorescence quantum yields a suitably chosen sample of DCA in acetonitrile was used as the reference (@r = 0.88 [2]). Data minimization processes were performed using the MINUITS procedure [lo] from the CERNLibrary.
3. Results 3. I. Flcrotxwence
quenching
qf DCA hv durene
Fluorescence quenching of DCA by durene is strongly dependent on temperature. The temperature dependence of the fluorescence quenching rate constant, k,, is presented in Table 1. The fluorescence quenching process of DCA by durene in acetonitrile is partially controlled by the diffusion. For the diffusion-controlled processes the quenching rate constant is time dependent. This time dependence results from the time dependence of the distance between the excited acceptor molecule and the donor molecule. The expression for the time-dependent quenching rate constant has
Table I Temperature behavior durene in acetonitrile
oC the fluorescence
‘/- 1K]
k,[lO”M-‘s
53
7.17 8.6X 9.75 I I.52 13.26 14.53 16.75
763 273 2x3 ‘0 3 303 3 I;
quenching
of DCA b!
the following
form [ 11,121:
k(r) = 47tN
k,,(l.)&., t)r’dr.
(1)
4 where LI is the distance of the closest approach of the donor and acceptor molecules. Ri’ is Avogadro’s number divided by 1000. k,,(r) is the distance-dependent electron transfer rate constant. and [‘(I’, t) is the time-dependent pair distribution function. p(r. t) is the solution of the diffusion equation: a,)@. t)$t
= D[d’.ldr’
+ (2:l.)d;dr]/)(r..
- k,,(r)p(r.
t)
t).
(2)
where D is the sum of the diffusion coeficients of the donor and acceptor molecules. This quantity may be numerically calculated by solving the Smoluchowski equation (2) in the Laplace domain with appropriate boundary conditions [ 11,13]. We adopted the Jortner equation for the distance-dependent electron transfer rate constant (source function) [13]. This equation has the following form: k,,(r) =
2 (‘~~h)I/(r,m)‘(4rri,(r)l;T) ,n= 0
-’ ?
x exp i - [AC,, + nzl& + i(r)]‘i4j,(r)liT
). (3)
where AC,, is the free energy of the electron transfer process, jb(r) is the solvent reorganization energy, and ha is the energy of the internal vibration. The 1/(r,m) is the modified electronic element and may be expressed as I/(r.nz)’
= I/?
exp[ - ~((r - 0)]cxp(
- S)S”‘!m (4)
‘1
@,,,
~~ lnsl
0.0113s 0.0089 0.00735 0.0065 0.005 0.00425 0.0037s
3.21 2.35 2.27 1.60 1.29 I .09
with S = ii,i’r?s2, where &,, is the internal reorganization energy and Y (scaling factor) describes the distance-dependence of the electronic coupling. For the evaluation of the solvent reorganization energy i(r) the Marcus equation was applied [14]:
(9
344
M. Mac et al. /Journal
qf Luminescence
where a, and a2 are the radii of the reactants, n is the refraction index of the solvent, and c, is the dielectric permittivity of the solvent. Fig. 2 represents the temperature dependence of the fluorescence quenching rate constants (open circles). The correlation line (solid line) is the result of the numerical calculation of the quenching rate constants based on the technique described in Refs. [ll, 121. The fitted parameters were the following: the distance of the closest approach, a, the electronic coupling at the distance of the closest approach, V(a), and the activation energy for diffusion, E,i,,. We assumed that the temperature dependence of the diffusion coefficients has the following form: D = D,exp[
- Ediff/RT].
r = 47rN’
ket(r)r2 dr.
(7)
s a Table 2 Parameters
used in calculations
3.2. Exciplex fluorescence acetonitrile
The fluorescence spectra of the mixtures of DCA and durene in acetonitrile reveal small fractions of
‘o.35 r;r. ,0,*5
. . . . . . ..
.._....
;..:‘......._ .__~~._i
,,
of the rate constants
1/T[K-‘1 Fig. 2. Temperature dependence of the rate constants of the fluorescence quenching of DCA by durene in acetonitrile. The correlation line (solid, a) represents the predictions given by the theory of diffusion-controlled reaction (see text). The broken line (b) represents the temperature behavior of the intrinsic electron transfer rate constants (without diffusion limitation).
of the fluorescence
quenching
Parameter
Symbol
Diffusion coefficient at 93 K Activation energy of the diffusion
D
DCA molecule radius Radius of durene Dielectric constant of acetonitrile at 293 K Temperature decrement of the dielectic constant Refraction index of acetonitrile at 293 K Temperature decrement of the refraction index Distance of closest approach Electronic coupling at closest approach distance Scaling factor Free energy of primary electron transfer step Energy of internal vibration “Parameters “Parameters
of DCA and durene in
(6)
The parameters used in our minimization procedure are collected in Table 2. The upper line in Fig. 2 represents the prediction of the averaged electron transfer rate constants calculated assuming an infinitely fast diffusion process. In this case the pair distribution function p(r, t) = 1. Thus Eq. (1) simplifies [15] to the following:
(k,,)
65 (1996) 341-348
taken from published sources. fitted to the experimental results
KM
of DCA by durene
in acetonitrile Value of the parameter 4.0 x 10-“m2s~’ 11.44 + 0.98/kJ M 9.6 $ 0.7 kJ Mm rr 4SA 4.oA 37.5 [I71 - 0.159311 1171 1.3441 [17] - 0.00048 [l?] 5.96 + 0.2 A 0.012 + 0.003 eV I.OA-’ - 0.27 eV 0.15 eV
jh
M. Mae et al. 1 Journal
of Luminescence
the exciplex fluorescence. The intensity of the exciplex emission grows up with increasing the durene concentration and strongly depends on temperature. The temperature dependence of the exciplex fluorescence quantum yields is presented in Table 1 and graphically in Fig. 3 (black circles and solid correlation line). The Arrhenius plot of ln(@_) against the inverse of temperature gives a perfectly straight line. From its slope the activation energy may be calculated. The activation energy is equal to 12.60 + 0.05 kJ M - I. Fig. 4 represents the extracted normalized exciplex emission of DCA and durene (0.150 M) in acetonitrile at 293 K. The correlation line is the predicted exciplex spectrum, calculated using the following formula [6-81: I(V) = Y3A i (S”,/m!) m= 0 x exp[ - (AC + m&2 + A + 1%)~/41.kT], where hi is the photon defined as
energy
and the factor
(8) A is
345
65 (I YM) H-348
0.5
0
-0.5
-1
-1.5 0.0031
0.0033
0.0035
0.0037
0.0039
1 /T[K.‘]
Fig. 3. Temperature dependencies of the exciplex quantum yields (black points and solid line). fluorescence lifetimes (open circles and broken line) and diffusion coeficients (triangles and solid line). The latter quantities are multlplied by a sultable factor for presentation purposes. The quantum yields of the exciplex are normahzed to the quantum yield at 293 K. (a) In(r,)=f(l’Tl;(b) In(@_,)=/(l’T):(c) -InlD)=f(l 7).
1 0.9
x exp( - S)[4ni.kT]-“2.
(8a)
The parameters EL(solvent reorganization energy), n and S were previously defined, p is the dipole moment of the exciplex and (/IV),,, is the energy at the maximum value of the exciplex intensity. It should be noted that the factor A vanishes upon normalization of the exciplex spectrum to its maximum value of the intensity. The fitted parameters are listed in Fig. 4. In Fig. 3 the temperature dependence of the exciplex lifetimes measured at ca. 580 nm is also shown. A typical fluorescence decay is given in Fig. 5. The fluorescence decays were essentially single exponentials with small contributions from the scattered light. We observe decreasing of the fluorescence lifetimes as temperature increases. The Arrhenius plot of ln(T,) against the inverse of temperature gives a straight line with a slope very similar to the slope of the dependence of ln(@,,,) versus 1iT. The calculated activation energy is equal to 12.1 + 1.4 kJ Mm’.
0.8 0.7
0.3 0.2 f-J.1
;:
1.
iJl
0 1 .40x104
1.60x104
1.80x104
2.00x104
wavenumber/cm-’
Fig. 4. Normalized exciplex fluorescence at 203 K. The solid line represents the prediction given by Eq. (XI. calculated with the parameters gwen in the figure.
3.3. Quenching
of the e.xciplex b.v lithium pet-chlorate
The next experiments were carried out to monitor the exciplex fluorescence quenching by lithium perchlorate in acetonitrile. We can observe the
346
M. Mac et al. /Journal
of’Luminewence
65 (1996) 341-34X
The SternVolmer constant is equal to 11.75 + 1.27 MP1 which corresponds to a fluorescence quenching constant of 7.34 + 0.79 x lo9 M- ’ s- I.
4. Discussion
-7
-8 0
150
100
50
200
[0.097 ns/ch]
channels
I
I
Fig. 5. Decay of the exciplex fluorescence 580 nm.
at 293 K measured
at
1.6
1.4
.
0
T
!LK ,j
i...
.,.,
.._..
; ..,...
..,
1 :
-0
,.i
1.2
-
.,
.,.
/..
..,,_.
..,.;
1
J 0
The fluorescence of DCA is strongly affected by the presence of durene. The AG value of the primary electron transfer step, which is responsible for the fluorescence quenching, indicates that the process is partially diffusion controlled. The detailed analysis of the temperature behavior of the fluorescence quenching rate constants yields some important parameters that characterize the electron transfer process as well as the diffusion process. A very important parameter that may be recovered from such a type of analysis is the activation energy of the diffusion coefficient of the reactants in acetonitrile. We have estimated this activation energy as 11.44 + 0.98 kJM_r. This quantity is very close to the activation energy of the exciplex fluorescence quantum yields or the exciplex lifetimes. Both values are close to the activation energy of diffusion equal to 9.6 + 0.7 kJM_’ which may be calculated using the Einstein formula [16] from the temperature dependence of the viscosity coefficients of acetonitrile [17]. It will be shown later that this agreement is not accidental. The solvent reorganization energy ;(a) related to the distance of the closest approach, a, and the geometrical and solvent parameters listed in Table 2, equals 0.48 eV at 293 K. This value is also very close to the quantity obtained from the analysis of the shape of the CT emission (i.e., 0.5 1 eV at 293 K). The distance of the closest approach equal to 5.96 + 0.2 seems to be reasonable as the average distance (center to center) between the two reactants in the exciplex. The energy of internal vibrations recovered from the analysis of the exciplex emission envelope equal to 0.35 eV is also very close to that obtained from the analysis of the temperature dependence of the fluorescence quenching rate constants (equal to 0.4eV). It is reasonable to assume that excess or loss of one electron in the reactants in the forward electron transfer step causes the same geometrical changes as those brought about by the backward electron transfer process. The value of I+ is in good
0.01
0.02
0.03
0.04
C/M Fig. 6. Stern-Volmer plot of the exciplex quenching perchlorate in acetonitrile at 293 K.
by lithium
Stern-Volmer quenching of the exciplex due to the added salt, though at the same time we can see that the perchlorate anions do not seem to influence the monomer fluorescence over a salt concentration range used in the experiments. The SternVolmer plot is shown in Fig. 6.
agreement with the estimations given by Gould et al. for the back electron transfer reactions between anion radicals of DCA and several radical cations of benzene derivatives [2]. The exciplex lifetime (TV)and its quantum yield (@_.) depend on the magnitude of the deactivation processes as pointed out in Fig. 1. The expressions for Qc,, and Qs are given below:
T’f= l,‘(li, + k_,, + k,“,,).
DCA and durene by lithium perchlorate. We found that this salt quenches the exciplex fluorescence. The quenching rate constant equals 7.34 x 10”~~’ M-’ and is less than the diffusion-controlled rate constant in acetonitrile that is taken usually as 2.0 x 10”’ Mm’ sm‘. Lithium perchlorate does not affect the fluorescence of DCA due to the high oxidation potential of the CIO, anion. The possible mechanism of quenching of the GRIP state by LiCIOl is presented as
(9) (DCA-,!Dur+)
where kr. k _c, and k,(>,, are the rate constants of the radiative electron transfer process, nonradiative ET. and solvation of the CRIP state to form the SSRIP state. respectively. It was shown by Gould et al. [6] that the rate constant of the back electron transfer process in the contact ionic pair at AG = - 2.79 eV attains the value of ca. 10’s~‘. A simple calculation of kf by combining the values of the exciplex lifetime and the exciplex quantum yields shows that k, should be of the order of IOhs- ‘. Thus. the nonradiative or radiative back electron transfer process cannot compete with the third deactivation route, namely with solvation of the GRIP to form the SSRIP. If the solvation process determines the deactivation rate constant of the exciplex then the activation energy of the exciplex fluorescence quantum yield or the exciplex lifetime should be approximately the same as that inferred from the temperature dependence of the diKusion coefficients:
=
I (k!i,lb exp
[ - E,i,-~~RT 1).
(10)
We showed that the activation energy of the diffusion calculated from the analysis of the temperature dependence of the quenching rate constants is approximately 11.44 kJ M - ‘. This value is very close to the values of the activation energy calculated from the Arrhenius dependencies of sf and Qexc. Thus, the dominant process of deactivation of the exciplex of DCA and durene in acetonitrile is the salvation of the CRIP state. It has been shown by Mattay and Vondenhof [I 81 that small inorganic cations accelerate the separation of the GRIP states. We performed the experiments with the quenching of the exciplex of
+ Lit $(DCA-,Li’)
+ Dur+.
The lithium cation is imbedded into the radical pair (CRIP) to form the nonfluorescent species. The reversibility of the formation of the complex of (DCA-:Li+) may be responsible for the quenching rate constant that is smaller than the appropriate diffusion-controlled rate constant.
5. Conclusions In this paper we reported the detailed analysis of the fluorescence quenching of DCA by durene in acetonitrile. The fluorescence quenching is partially controlled by the diffusion. By solving the diffusion equation (Smoluchowski equation) with the source function given by the Jortner formula for the electron transfer rate constant we reproduced the values of the intrinsic electron transfer rate constants. The estimated activation energy of the diffusion corresponds quite well to the activation energy of the exciplex lifetime or the exciplex fluorescence quantum yield indicating that the dominant deactivation pathway of the exciplex is the solvation of the CRIP state. The quenching of the CRIP state by lithium cations is probably related to the reversible formation of the (DCA -YLi’) nonfluorescing complex.
Acknowledgements We thank Dr. Janusz Golus for the purification of durene. This work has been financially supported by the Polish Committee for Scientific Research.
348
M. Mac et al. /Journal
of’Luminescence 65 (1996) 341-348
References Helv. Chim. Cl1 E. Vauthey, P. Suppan and E. Haselbach, Acta 71 (1988) 93. PI I.R. Gould, D. Ege, J.E. Moser and S. Farid, J. Am. Chem. Sot. 112 (1990) 4290. c31 K. Kikuchi, T. Katagiri, T. Niwa, Y. Takahashi, T. Suzuki, H. Ikeda and T. Miyashi, Chem. Phys. Lett. 193 (1992) 155. [41 M. Mac, J. Najbar and J. Wirz. Chem. Phys. Lett. 35 (1995) 187. H. Ikeda, T. Miyashi c51 K. Kikuchi, T. Niwa, Y. Takahashi, and M. Hoshi, Chem. Phys. Lett. 173 (1990) 421. C61 I.R. Gould, R.H. Young, R.E. Moody and S. Farid, J. Phys. Chem. 95 (1991) 2068. c71 R.A. Marcus, J. Phys. Chem. 93 (1989) 3078. and S. Farid. J. Photochem. Photobiol. I?31 JR. Gould A Chem. 65 (1992) 133.
[9] M. Mac, W. Jarzeba and J. Najbar, J. Lumin. 37 (1987) 139. [lo] F. James and M. Ross, Comput. Phys. Commun. 20 (1980) 29. [l l] J. Najbar, J. Phys. Chem. 94 (1990) 367. 1121 J. Najbar and M. Mac, J. Chem. Sot. Faraday Trans. 87 (1991) 1523. 1131 J. Jortner, J. Chem. Phys. 64 (1976) 4860. [14] R.A. Marcus, J. Chem. Phys. 24 (1956) 966. [15] M. Mac and J. Wirz, Chem. Phys. Lett. 211 (1993) 20. 1161 J.B. Birks, Organic Molecular Photophysics, Vol. I (Wiley, London, 1973) ch. 8. 1171 Landolt-Bornstein, Zahlenwerte und Funktionen aus Physik-ChemieeAstronomieeGeophysik und Technik, Part 5, 6 and 8 (Springer, Berlin, 1959). [18] J. Mattay, and M. Vondenhof, in: Topics in Current Chemistry, Vol. 159 (Springer, Berlin, 1991) pp. 119-255.
OF
LUMINESCENCE ELSEVIER
Journal
of Luminescence
65 (1996) M-348
Influence of temperature on fluorescence quenching of 9, lo-dicyanoanthracene by durene in acetonitrile Marek Mac*, Piotr Kwiatkowski, Andrzej M. Turek Received
31 August
1995: accepted
13 September
1995
Abstract The effect of temperature on the fluorescence quenching of 9,10-dicyanoanthracene has been investigated. The fluorescence quenching in the system is accompanied
(DCA) by durene in acetonitrile by a weak exciplex emission. The
exciplex fluorescence quantum yields and exciplex lifetimes depend strongly on temperature and the exciplex emission is efficiently quenched by lithium perchlorate. The most important pathway of deactivation of the contact ionic pair (GRIP) is salvation of the CRIP. The structure of the CRIP has been discussed on the basis of the analysis of the exciplex fluorescence spectra as well as the temperature dependence of the fluorescence quenching rate constants.
1. Introduction Fluorescence quenching of 9,10-dicyanoanthracene (DCA) by organic neutral donors in acetonitrile has been intensively investigated [l-4]. The presence of the radical anions of DCA and the radical cations of the donors forming in the primary electron transfer step was observed in the transient absorption spectra or in the photocurrent experiments. Usually, only free radicals are the transient intermediates observed in the nanosecond or microsecond flash photolysis experiments. The magnitude of the charge separation quantum yields depends on the back electron transfer rate constants. There are some reports on the formation of the emissive species of DCA and several organic
*Co)-responding
author
0022-2313~96~$15.00 ( 1996 SD/ 0022-23 13(95)00079-8
donors in acetonitrile [5,6]. Such species arc called exciplexes (excited complexes). Usually. the exciplexes were observed in the appropriate donor sacceptor systems in weakly polar solvents. Recent papers of Gould et al. [6] showed the exciplex emission in the systems containing strong electron acceptors such as 9.1 O-dicyanoanthracene (DCA) or 2,6,9.10-tetracyanoanthracene (TCA) and electron donors (alkyl derivatives of benzene) in acetonitrile. The mechanistic scheme that describes the processes following the electron transfer fluorescence quenching has been presented [6]. This is shown in Fig. 1. Electron transfer fluorescence quenching leads to the solvent separated radical ion pair (SSRIP = A-(S)D+) or to the contact radical ion pair (CRIP = A-D+). The two radical species differ from each other in electronic coupling, which is much higher in the latter case. This factor is responsible for the fluorescence resulting from the CRIP.
Elsevier Science B.V. All rights reserved
342
M. Mac et al. J Journal
c-
A*+D
of Luminescence 65
7
hv hv . -
4kCetlcp AD
AWD
Fig. 1. Photophysical processes following fluorophore in the presence of the quencher solvents.
A+Dl
the excitation of molecules in polar
Excitation of the charge transfer complex (AD) leads directly to the CRIP. A higher efficiency of the emission from the CT state observed after excitation of the AD complexes compared to that of the CT state produced by diffusive quenching of A* by D indicates the importance of the production of SSRIP without the exciplex intermediate. Both species (SSRIP and CRIP) may dissociate into free radicals (A- + D+) which are usually observed in the nanosecond and microsecond transient absorption experiments. It is difficult to differentiate between the CRIP and SSRIP states on the basis of the transient absorption measurements due to similarity of their absorption spectra. In general, the exciplex is a mixture of the states (A-D+ Z$ A*D). If the admixture of the locally excited state A*D is negligible, the term exciplex is equivalent to the CRIP state. It is assumed that in polar solvents such as acetonitrile the mixing of other states than CT is negligible [6]. Thus, in such systems the shape of the emission envelope may be analyzed in terms of Marcus theory [7]. Such a type of analysis provides the values of the important parameters characterizing the electron transfer process, i.e., the free energy of the back electron transfer process (AG) and the reorganization energies (ni, and j,) [6,8] as presented in papers of Gould et al. [6]. It has been found that the fluorescence quantum yields of the excited DA complexes of TCA with methyl derivatives of benzene are very low due to very fast other deactivation processes such as nonradiative back electron transfer process (k, _ +,,) and solvation (k,,,,). The fluorescence quantum yields of the DCA exciplexes, formed in the colli-
11996) 341-348
sional way from the encounter complex (A*(S)D) are much higher due to the larger reduction potential of DCA as compared with TCA (Ered @CA) = - 0.91 eV/SCE, Ered(TCA) = - 0.44 eV/SCE) which makes the AG more negative in the case of DCA exciplexes. Thus, the nonradiative back electron transfer in CRIP is slower in the DCA complexes (inverted Marcus region). The aim of this work is to analyze the temperature behavior of the exciplexes of DCA and durene in acetonitrile using steady-state and pulse fluorescence techniques.
2. Experimental DCA (Tokyo Kasei) was crystallized from toluene. Durene (Koch-light Laboratories) was sublimed under vacuum. Acetonitrile of spectroscopic grade (Fluka) was used without purification. The absorption spectra of the mixture of DCA and durene were strictly additive indicating no ground state complexation in the quencher concentration used in experiments (up to 0.145 M). The steady-state measurements were performed using a conventional spectrolluorimeter with a cooled photomultiplier EMI 955 8B operating in a single photon counting mode. For excitation the 405 nm Hg line was selected. The sample was held in a high and low temperature cryostat (Oxford Instruments, DN1704). The fluorescence spectra were corrected for the spectral sensitivity of the detecting system. The fluorescence decay functions were recorded using the time-correlated single photon counting arrangement equipped with a Hamamatsu photomultiplier (R3809U MCP-PMT), with a nitrogen flash lamp as an excitation source (half-width ca. 1.2 ns) [9]. For excitation the 405 nm line was selected. The fluorescence lifetimes of the mixtures of DCA and durene were calculated by fitting the experimentally recorded fluorescence decay functions to the convolution of a monoexponential decay function and excitation profile. Stern-Volmer plots were constructed from the lifetimes of the perturbed DCA samples. The SternVolmer plots were linear within the quencher concentration
range used in the experiments. The changes of the concentration of the perturber due to the changes of temperature were taken into account in the calculations of SternPVolmer rate constants. The decays of the exciplex fluorescence were measured at 580 nm. The exciplex fluorescence spectra of DCA and durene in acetonitrile were extracted from the emission spectra after subtracting from them the residual monomer fluorescence multiplied by an appropriate factor. For the estimation of the exciplex fluorescence quantum yields a suitably chosen sample of DCA in acetonitrile was used as the reference (@r = 0.88 [2]). Data minimization processes were performed using the MINUITS procedure [lo] from the CERNLibrary.
3. Results 3. I. Flcrotxwence
quenching
qf DCA hv durene
Fluorescence quenching of DCA by durene is strongly dependent on temperature. The temperature dependence of the fluorescence quenching rate constant, k,, is presented in Table 1. The fluorescence quenching process of DCA by durene in acetonitrile is partially controlled by the diffusion. For the diffusion-controlled processes the quenching rate constant is time dependent. This time dependence results from the time dependence of the distance between the excited acceptor molecule and the donor molecule. The expression for the time-dependent quenching rate constant has
Table I Temperature behavior durene in acetonitrile
oC the fluorescence
‘/- 1K]
k,[lO”M-‘s
53
7.17 8.6X 9.75 I I.52 13.26 14.53 16.75
763 273 2x3 ‘0 3 303 3 I;
quenching
of DCA b!
the following
form [ 11,121:
k(r) = 47tN
k,,(l.)&., t)r’dr.
(1)
4 where LI is the distance of the closest approach of the donor and acceptor molecules. Ri’ is Avogadro’s number divided by 1000. k,,(r) is the distance-dependent electron transfer rate constant. and [‘(I’, t) is the time-dependent pair distribution function. p(r. t) is the solution of the diffusion equation: a,)@. t)$t
= D[d’.ldr’
+ (2:l.)d;dr]/)(r..
- k,,(r)p(r.
t)
t).
(2)
where D is the sum of the diffusion coeficients of the donor and acceptor molecules. This quantity may be numerically calculated by solving the Smoluchowski equation (2) in the Laplace domain with appropriate boundary conditions [ 11,13]. We adopted the Jortner equation for the distance-dependent electron transfer rate constant (source function) [13]. This equation has the following form: k,,(r) =
2 (‘~~h)I/(r,m)‘(4rri,(r)l;T) ,n= 0
-’ ?
x exp i - [AC,, + nzl& + i(r)]‘i4j,(r)liT
). (3)
where AC,, is the free energy of the electron transfer process, jb(r) is the solvent reorganization energy, and ha is the energy of the internal vibration. The 1/(r,m) is the modified electronic element and may be expressed as I/(r.nz)’
= I/?
exp[ - ~((r - 0)]cxp(
- S)S”‘!m (4)
‘1
@,,,
~~ lnsl
0.0113s 0.0089 0.00735 0.0065 0.005 0.00425 0.0037s
3.21 2.35 2.27 1.60 1.29 I .09
with S = ii,i’r?s2, where &,, is the internal reorganization energy and Y (scaling factor) describes the distance-dependence of the electronic coupling. For the evaluation of the solvent reorganization energy i(r) the Marcus equation was applied [14]:
(9
344
M. Mac et al. /Journal
qf Luminescence
where a, and a2 are the radii of the reactants, n is the refraction index of the solvent, and c, is the dielectric permittivity of the solvent. Fig. 2 represents the temperature dependence of the fluorescence quenching rate constants (open circles). The correlation line (solid line) is the result of the numerical calculation of the quenching rate constants based on the technique described in Refs. [ll, 121. The fitted parameters were the following: the distance of the closest approach, a, the electronic coupling at the distance of the closest approach, V(a), and the activation energy for diffusion, E,i,,. We assumed that the temperature dependence of the diffusion coefficients has the following form: D = D,exp[
- Ediff/RT].
r = 47rN’
ket(r)r2 dr.
(7)
s a Table 2 Parameters
used in calculations
3.2. Exciplex fluorescence acetonitrile
The fluorescence spectra of the mixtures of DCA and durene in acetonitrile reveal small fractions of
‘o.35 r;r. ,0,*5
. . . . . . ..
.._....
;..:‘......._ .__~~._i
,,
of the rate constants
1/T[K-‘1 Fig. 2. Temperature dependence of the rate constants of the fluorescence quenching of DCA by durene in acetonitrile. The correlation line (solid, a) represents the predictions given by the theory of diffusion-controlled reaction (see text). The broken line (b) represents the temperature behavior of the intrinsic electron transfer rate constants (without diffusion limitation).
of the fluorescence
quenching
Parameter
Symbol
Diffusion coefficient at 93 K Activation energy of the diffusion
D
DCA molecule radius Radius of durene Dielectric constant of acetonitrile at 293 K Temperature decrement of the dielectic constant Refraction index of acetonitrile at 293 K Temperature decrement of the refraction index Distance of closest approach Electronic coupling at closest approach distance Scaling factor Free energy of primary electron transfer step Energy of internal vibration “Parameters “Parameters
of DCA and durene in
(6)
The parameters used in our minimization procedure are collected in Table 2. The upper line in Fig. 2 represents the prediction of the averaged electron transfer rate constants calculated assuming an infinitely fast diffusion process. In this case the pair distribution function p(r, t) = 1. Thus Eq. (1) simplifies [15] to the following:
(k,,)
65 (1996) 341-348
taken from published sources. fitted to the experimental results
KM
of DCA by durene
in acetonitrile Value of the parameter 4.0 x 10-“m2s~’ 11.44 + 0.98/kJ M 9.6 $ 0.7 kJ Mm rr 4SA 4.oA 37.5 [I71 - 0.159311 1171 1.3441 [17] - 0.00048 [l?] 5.96 + 0.2 A 0.012 + 0.003 eV I.OA-’ - 0.27 eV 0.15 eV
jh
M. Mae et al. 1 Journal
of Luminescence
the exciplex fluorescence. The intensity of the exciplex emission grows up with increasing the durene concentration and strongly depends on temperature. The temperature dependence of the exciplex fluorescence quantum yields is presented in Table 1 and graphically in Fig. 3 (black circles and solid correlation line). The Arrhenius plot of ln(@_) against the inverse of temperature gives a perfectly straight line. From its slope the activation energy may be calculated. The activation energy is equal to 12.60 + 0.05 kJ M - I. Fig. 4 represents the extracted normalized exciplex emission of DCA and durene (0.150 M) in acetonitrile at 293 K. The correlation line is the predicted exciplex spectrum, calculated using the following formula [6-81: I(V) = Y3A i (S”,/m!) m= 0 x exp[ - (AC + m&2 + A + 1%)~/41.kT], where hi is the photon defined as
energy
and the factor
(8) A is
345
65 (I YM) H-348
0.5
0
-0.5
-1
-1.5 0.0031
0.0033
0.0035
0.0037
0.0039
1 /T[K.‘]
Fig. 3. Temperature dependencies of the exciplex quantum yields (black points and solid line). fluorescence lifetimes (open circles and broken line) and diffusion coeficients (triangles and solid line). The latter quantities are multlplied by a sultable factor for presentation purposes. The quantum yields of the exciplex are normahzed to the quantum yield at 293 K. (a) In(r,)=f(l’Tl;(b) In(@_,)=/(l’T):(c) -InlD)=f(l 7).
1 0.9
x exp( - S)[4ni.kT]-“2.
(8a)
The parameters EL(solvent reorganization energy), n and S were previously defined, p is the dipole moment of the exciplex and (/IV),,, is the energy at the maximum value of the exciplex intensity. It should be noted that the factor A vanishes upon normalization of the exciplex spectrum to its maximum value of the intensity. The fitted parameters are listed in Fig. 4. In Fig. 3 the temperature dependence of the exciplex lifetimes measured at ca. 580 nm is also shown. A typical fluorescence decay is given in Fig. 5. The fluorescence decays were essentially single exponentials with small contributions from the scattered light. We observe decreasing of the fluorescence lifetimes as temperature increases. The Arrhenius plot of ln(T,) against the inverse of temperature gives a straight line with a slope very similar to the slope of the dependence of ln(@,,,) versus 1iT. The calculated activation energy is equal to 12.1 + 1.4 kJ Mm’.
0.8 0.7
0.3 0.2 f-J.1
;:
1.
iJl
0 1 .40x104
1.60x104
1.80x104
2.00x104
wavenumber/cm-’
Fig. 4. Normalized exciplex fluorescence at 203 K. The solid line represents the prediction given by Eq. (XI. calculated with the parameters gwen in the figure.
3.3. Quenching
of the e.xciplex b.v lithium pet-chlorate
The next experiments were carried out to monitor the exciplex fluorescence quenching by lithium perchlorate in acetonitrile. We can observe the
346
M. Mac et al. /Journal
of’Luminewence
65 (1996) 341-34X
The SternVolmer constant is equal to 11.75 + 1.27 MP1 which corresponds to a fluorescence quenching constant of 7.34 + 0.79 x lo9 M- ’ s- I.
4. Discussion
-7
-8 0
150
100
50
200
[0.097 ns/ch]
channels
I
I
Fig. 5. Decay of the exciplex fluorescence 580 nm.
at 293 K measured
at
1.6
1.4
.
0
T
!LK ,j
i...
.,.,
.._..
; ..,...
..,
1 :
-0
,.i
1.2
-
.,
.,.
/..
..,,_.
..,.;
1
J 0
The fluorescence of DCA is strongly affected by the presence of durene. The AG value of the primary electron transfer step, which is responsible for the fluorescence quenching, indicates that the process is partially diffusion controlled. The detailed analysis of the temperature behavior of the fluorescence quenching rate constants yields some important parameters that characterize the electron transfer process as well as the diffusion process. A very important parameter that may be recovered from such a type of analysis is the activation energy of the diffusion coefficient of the reactants in acetonitrile. We have estimated this activation energy as 11.44 + 0.98 kJM_r. This quantity is very close to the activation energy of the exciplex fluorescence quantum yields or the exciplex lifetimes. Both values are close to the activation energy of diffusion equal to 9.6 + 0.7 kJM_’ which may be calculated using the Einstein formula [16] from the temperature dependence of the viscosity coefficients of acetonitrile [17]. It will be shown later that this agreement is not accidental. The solvent reorganization energy ;(a) related to the distance of the closest approach, a, and the geometrical and solvent parameters listed in Table 2, equals 0.48 eV at 293 K. This value is also very close to the quantity obtained from the analysis of the shape of the CT emission (i.e., 0.5 1 eV at 293 K). The distance of the closest approach equal to 5.96 + 0.2 seems to be reasonable as the average distance (center to center) between the two reactants in the exciplex. The energy of internal vibrations recovered from the analysis of the exciplex emission envelope equal to 0.35 eV is also very close to that obtained from the analysis of the temperature dependence of the fluorescence quenching rate constants (equal to 0.4eV). It is reasonable to assume that excess or loss of one electron in the reactants in the forward electron transfer step causes the same geometrical changes as those brought about by the backward electron transfer process. The value of I+ is in good
0.01
0.02
0.03
0.04
C/M Fig. 6. Stern-Volmer plot of the exciplex quenching perchlorate in acetonitrile at 293 K.
by lithium
Stern-Volmer quenching of the exciplex due to the added salt, though at the same time we can see that the perchlorate anions do not seem to influence the monomer fluorescence over a salt concentration range used in the experiments. The SternVolmer plot is shown in Fig. 6.
agreement with the estimations given by Gould et al. for the back electron transfer reactions between anion radicals of DCA and several radical cations of benzene derivatives [2]. The exciplex lifetime (TV)and its quantum yield (@_.) depend on the magnitude of the deactivation processes as pointed out in Fig. 1. The expressions for Qc,, and Qs are given below:
T’f= l,‘(li, + k_,, + k,“,,).
DCA and durene by lithium perchlorate. We found that this salt quenches the exciplex fluorescence. The quenching rate constant equals 7.34 x 10”~~’ M-’ and is less than the diffusion-controlled rate constant in acetonitrile that is taken usually as 2.0 x 10”’ Mm’ sm‘. Lithium perchlorate does not affect the fluorescence of DCA due to the high oxidation potential of the CIO, anion. The possible mechanism of quenching of the GRIP state by LiCIOl is presented as
(9) (DCA-,!Dur+)
where kr. k _c, and k,(>,, are the rate constants of the radiative electron transfer process, nonradiative ET. and solvation of the CRIP state to form the SSRIP state. respectively. It was shown by Gould et al. [6] that the rate constant of the back electron transfer process in the contact ionic pair at AG = - 2.79 eV attains the value of ca. 10’s~‘. A simple calculation of kf by combining the values of the exciplex lifetime and the exciplex quantum yields shows that k, should be of the order of IOhs- ‘. Thus. the nonradiative or radiative back electron transfer process cannot compete with the third deactivation route, namely with solvation of the GRIP to form the SSRIP. If the solvation process determines the deactivation rate constant of the exciplex then the activation energy of the exciplex fluorescence quantum yield or the exciplex lifetime should be approximately the same as that inferred from the temperature dependence of the diKusion coefficients:
=
I (k!i,lb exp
[ - E,i,-~~RT 1).
(10)
We showed that the activation energy of the diffusion calculated from the analysis of the temperature dependence of the quenching rate constants is approximately 11.44 kJ M - ‘. This value is very close to the values of the activation energy calculated from the Arrhenius dependencies of sf and Qexc. Thus, the dominant process of deactivation of the exciplex of DCA and durene in acetonitrile is the salvation of the CRIP state. It has been shown by Mattay and Vondenhof [I 81 that small inorganic cations accelerate the separation of the GRIP states. We performed the experiments with the quenching of the exciplex of
+ Lit $(DCA-,Li’)
+ Dur+.
The lithium cation is imbedded into the radical pair (CRIP) to form the nonfluorescent species. The reversibility of the formation of the complex of (DCA-:Li+) may be responsible for the quenching rate constant that is smaller than the appropriate diffusion-controlled rate constant.
5. Conclusions In this paper we reported the detailed analysis of the fluorescence quenching of DCA by durene in acetonitrile. The fluorescence quenching is partially controlled by the diffusion. By solving the diffusion equation (Smoluchowski equation) with the source function given by the Jortner formula for the electron transfer rate constant we reproduced the values of the intrinsic electron transfer rate constants. The estimated activation energy of the diffusion corresponds quite well to the activation energy of the exciplex lifetime or the exciplex fluorescence quantum yield indicating that the dominant deactivation pathway of the exciplex is the solvation of the CRIP state. The quenching of the CRIP state by lithium cations is probably related to the reversible formation of the (DCA -YLi’) nonfluorescing complex.
Acknowledgements We thank Dr. Janusz Golus for the purification of durene. This work has been financially supported by the Polish Committee for Scientific Research.
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M. Mac et al. /Journal
of’Luminescence 65 (1996) 341-348
References Helv. Chim. Cl1 E. Vauthey, P. Suppan and E. Haselbach, Acta 71 (1988) 93. PI I.R. Gould, D. Ege, J.E. Moser and S. Farid, J. Am. Chem. Sot. 112 (1990) 4290. c31 K. Kikuchi, T. Katagiri, T. Niwa, Y. Takahashi, T. Suzuki, H. Ikeda and T. Miyashi, Chem. Phys. Lett. 193 (1992) 155. [41 M. Mac, J. Najbar and J. Wirz. Chem. Phys. Lett. 35 (1995) 187. H. Ikeda, T. Miyashi c51 K. Kikuchi, T. Niwa, Y. Takahashi, and M. Hoshi, Chem. Phys. Lett. 173 (1990) 421. C61 I.R. Gould, R.H. Young, R.E. Moody and S. Farid, J. Phys. Chem. 95 (1991) 2068. c71 R.A. Marcus, J. Phys. Chem. 93 (1989) 3078. and S. Farid. J. Photochem. Photobiol. I?31 JR. Gould A Chem. 65 (1992) 133.
[9] M. Mac, W. Jarzeba and J. Najbar, J. Lumin. 37 (1987) 139. [lo] F. James and M. Ross, Comput. Phys. Commun. 20 (1980) 29. [l l] J. Najbar, J. Phys. Chem. 94 (1990) 367. 1121 J. Najbar and M. Mac, J. Chem. Sot. Faraday Trans. 87 (1991) 1523. 1131 J. Jortner, J. Chem. Phys. 64 (1976) 4860. [14] R.A. Marcus, J. Chem. Phys. 24 (1956) 966. [15] M. Mac and J. Wirz, Chem. Phys. Lett. 211 (1993) 20. 1161 J.B. Birks, Organic Molecular Photophysics, Vol. I (Wiley, London, 1973) ch. 8. 1171 Landolt-Bornstein, Zahlenwerte und Funktionen aus Physik-ChemieeAstronomieeGeophysik und Technik, Part 5, 6 and 8 (Springer, Berlin, 1959). [18] J. Mattay, and M. Vondenhof, in: Topics in Current Chemistry, Vol. 159 (Springer, Berlin, 1991) pp. 119-255.