2250
J. Phys. Chem. 1995,99, 2250-2253
Transient Charge Transfer Absorption Bands as Probes of Ion-Pairing Dynamics and Energetics Piotr Piotrowiak,* Renata KobetiC, Timothy Schatz, and Gina Strati Department of Chemistry, University of New Orleans, New Orleans, Louisiana 70148 Received: November 28, I994@ Very large time-dependent spectral blue shifts (up to over 7400 cm-l) induced by ion pairing were observed for long-lived photoinduced charge-separated states of two probe molecules: 4-amino-4'-nitrobiphenyl (pANBP) and 4-amino-4'-nitroterphenyl @-A" These ). shifts are analogous to the well-known time-dependent fluorescence shifts (TDFS) of transient CT states, which are attributed to nonequilibrium dipolar solvation in polar media as well as to ionic atmosphere relaxation in solutions of electrolytes. It is demonstrated that the dynamic spectral shifts of transient CT absorption bands can be utilized as sensitive probes of ion-pairing processes and that they offer significant advantages over the traditional fluorescent probes. The long lifetime of charge-separated excited triplet states (e.g., >3 ,us for p-ANTP) allows time-resolved study of electrolyte dynamics in highly dilute solutions ( 100 mM salt) and limit the approach to highly polar solvents.
Introduction The renewed interest in the role of electrolytes and counterions in intramolecular charge transfer reactions has lead to a number of important including the recent finding of the importance of ionic relaxation in the final stabilization of the photoinduced charge-separated species in bacterial photosynthetic reactions center^.^ The major contribution to the understanding of the dynamics of ionic atmosphere relaxation and the ion association processes following the creation of a charge-separated species in solution of an electrolyte (represented pictorially in Scheme 1)6 was provided by the studies of time-dependent fluorescence shifts of a suitable probe molecule.1*2 In this work we describe a significant variation of this concept, in which instead of following the dynamic red shift of the CTfluorescence, we have monitored the evolution of the time-dependent blue shift of the transient triplet CT absorption. The probe molecules of choice were p-aminonitrobiphenyl (p-ANBP) and p-aminonitroterphenyl (p-ANTP) (Figure l), whose transient triplet CT state absorption bands were investigated in THF solutions of chemically inert electrolytes. These are well-known molecules whose various properties, including nonlinear optical susceptibilities, have been previously investigated by other Transient microwave conductivity measurements established the dipole moment of the CT triplet state of p-ANBP to be approximately 50 D, corresponding to a nearly complete charge separation, with the positive charge localized at the amine end and the negative charge at the nitro end of the mole~ule.~
n
w
D -
The primary advantage of the presented transient triplet absorption approach over the more traditional fluorescent probes results from the vast difference between the lifetimes of the @
Abstract published in Advance ACS Abstracts, February 15, 1995.
Figure 1. Probe molecules 4-amino-4'-nitrobiphenyl (p-ANBP) and 4-amino-4'-nitroterphenyl (p-ANTP).
relevant excited states. Commonly available organic charge transfer dyes, such as coumarin, which was used in the majority of the earlier work in this area, exhibit fluorescence lifetimes in the range 1-5 ns.1-2 If a measurable red shift of fluorescence is to be detected, the specific pairing with the ions, or the ionic atmosphere relaxation, has to occur at a faster, or at least comparable, time scale. Unless the concentration of an electrolyte is of the order of 0.1 M or more, the ionic relaxation process is unable to compete with the decay of the probing state (assuming a diffusion-limited association process, with a pseudounimolecular rate of k,,,, = 1 x 1O1OM-' s-' and kfl= 1 x lo9 s-'). Such high electrolyte concentrations can be achieved only in highly polar media, and the best results were usually obtained in acetonitrile, E = 36. Unfortunately, the high dielectric constant of the solvent attenuates all Coulombic interactions and consequently lowers the magnitude of the observed spectral shift. Indeed, the largest electrolyte-induced shifts measured using fluorescent probes do not exceed 2000 cm-I and are smaller than corresponding shifts observed upon transition from a weakly to a highly polar solvent.2 Thanks to the much longer lifetime of the triplet charge transfer states (typically several microseconds), the process of association with counterions can be followed to a full equilibrium even in solutions containing only submillimolar concentrations of an electrolyte. Therefore, direct spectroscopic measurements of dynamics and energetics of ion pairing can be extended to weakly polar solvents such as tetrahydrofuran (E = 7.6), diethyl ether (E = 4.3), and dioxane (E = 2.2). Much larger spectral shifts and stabilization (reorganization) energies due to ion pairing can be expected as a result of the low dielectric constant of the medium. Experimental Section The p-aminonitrobiphenyl and p-aminonitroterphenyl were prepared via the classical route of double nitration of biphenyl
0022-3654/95/2099-2250$09.00/00 1995 American Chemical Society
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J. Phys. Chem., Vol. 99, No. 8, 1995 2251
or terphenyl to the conesponding p-dinitro derivative, followed by partial reduction to the p-aminonitro compound.10 The compounds were purified by recrystallization. They were carefully dried under vacuum prior to use. Lithium chloride (Aldrich, ACS Reagent, 99%) and tetrabutylammonium bromide (Aldrich, 99%) were used without further purification. The LiCl was vacuum dried. The samples containing 0.1 mh4 of the probe compound and a variable amount of one of the salts in THF (EM Science, dried over MgS04 and distilled over LiAlH4) were degassed by 3-5 freeze-pump-thaw cycles. The transient absorption experiments were performed using a nanosecond time-resolved spectrometer built around a Continuum NY61-10 Nd:YAG laser and a Tektronix SCDlOOO ultrafast digitizer. The third harmonic output of the Nd:YAG (355 nm), with a typical pulse energy of approximately 30 mJ and a pulse length of 7 ns, was used as the excitation source. A Hamamatsu Super-Quiet flash lamp served as the source of analyzing light. The transients were detected using a photomultiplier tube (Hamamatsu R928) in the visible range and a high-speed germanium photodiode (EG&G Judson J- 16-R01MHS) in the near-IR. Visible blocking cutoff filters were used during the near-IR measurements. The fluorescence spectra were collected using a Perkin-Elmer LS-5B fluorimeter. Solutions (0.1 mM) of the probe compounds in THF were used. The excitation wavelength was 355 nm. All measurements were performed at room temperature.
0.6 0.5
-0.1
1
3\(/
550
750
1150
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Wavelength [nml
\T
.
= 1.4 P
Results and Discussion Excitation of p-ANBP and p-ANTP with 355 nm light leads to a local excited state, most likely localized on the "nitrobenzene half' of the molecule. The lifetime of the fluorescent state, which depending on the medium polarity has a variable degree of a charge transfer character, is on the order of 3 nsS7Efficient intersystem crossing to a charge-separated triplet state occurs at the same time scale and from the viewpoint of our experiment can be treated as instantaneous. As a result, the interesting question of the primary pathway of the internal conversion process, i.e., the sequence of events (1) LE singlet state CT singlet state CT triplet state or (2) LE singlet state LE triplet state CT triplet state, is not important in the interpretation of the presented results. Nevertheless, it will be addressed in the future. The triplet CT states of p-ANBP and p-ANTP exhibit strong absorption bands in the red to near-IR portion of the spectrum (Figure 2a). The spectra are strongly solvatochromic." In THF the maxima are I,, = 710 nm and A,, = 990 nm, with approximate extinction coefficients of €710 x 25 000 and € 9 x 40 000, respectively. The triplet yields were not studied in detail yet. The measured single-exponential decay rates are 7.1 x lo5 s-l for p-ANBP and 3.1 x lo5 s-l for p-ANTP (Figure 2b). Upon addition of millimolar quantities of TBABr or LiC1, a dynamic blue shift of the triplet CT absorption due to ion pairing with the introduced electrolyte is observed (Figure 3). Immediately after the laser pulse the spectrum is identical with the one measured in neat THF. (Compare the first trace in Figure 3 with the appropriate trace in Figure 2a.) As the delay increases, the spectrum evolves to a new equilibrium position characterized by a much shorter wavelength I,=. The most dramatic shift was observed for p-ANTp in the presence of LiCl, in which case the I,, changes from 990 nm in the neat THF to 590 nm (Figure 3b) in the fully associated form, giving Av = 7440 cm-', Le., nearly 1 eV. This is nearly 4 times more than the largest electrolyte-induced shift observed using fluo-
--
0.00€+00 1.00E.06
2.00E-08
3.WE-06
4"-08
5.00E-08
Time [sl
Figure 2. (a, top) Transient triplet absorption spectra of 0.1 mM p-ANBP (left curve) andp-ANTP (right curve) in THF, excited at 355 nm. (Both transients exhibit bleaching at short wavelengths.) (b, bottom) Normalized CT triplet state decay profiles of p-ANBP at 7 10 nm (bottom curve) and p-ANTP at 990 nm (top curve). Note the excellent agreement with the single-exponential fits.
--
~
rescent probese2 The smallest shift of Av = 2310 cm-' was observed for p-ANBP in the presence of TBABr (Figure 3b). The magnitudes of the spectral shifts are gathered in Table 1. In all cases the equilibrium appears to be completely shifted toward the associated form, and no residual peak at the neat THF value of Imaxcan be found in the spectrum (Figure 3b). This is consistent with the pairing stabilization energy which even in the case of the smallest shift of 2310 cm-' is e6.5 kcal/mol, suggesting that the charge-separated species are fully associated with counterions already at millimolar salt concentrations. The observed increase of the shift with decreasing ion size follows the intuitive trend and is in agreement with the earlier results.2 The fact that the spectral shift is consistently larger in the case of p-ANTP than for p-ANBP can be also easily rationalized on the basis of the different length of the probe molecules: the presence of a counterion at one end of the charge-separated species stabilizes electrostatically the side of attachment; however, it destabilizes, by a smaller amount, the other, oppositely charged end of the probe molecule. In the first-order approximation the magnitude of this destabilization is inversely proportional to the length of the probe; Le., it is %30% smaller in the case of the longer p-ANTP. As expected, the fluorescence spectra of p-ANTP and p-ANBP, including the long wavelength CT band, remain completely unchanged in the presence of similar concentrations of an electrolyte, proving that no significant ion pairing with the charge-separated singlet state can occur during its lifetime. Moreover, the invariance of the fluorescence spectra indicates
2252 J. Phys. Chem., Vol. 99, No. 8, 1995
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1
2.00E-07 350
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1350
7.00E-07
1.20E-06
1.70E-06
Time (s]
1550
Wavelength [nm]
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Time (s] 3 50
550
950
750
1150
1350
1550
Wavelength (nm]
Figure 3. (a, top) Time evolution of the p-ANTP transient CT triplet spectrum in THF containing 1 mM TBABr. The delay after the laser pulse is indicated in the plot. (b, bottom) Fully equilibrated transient CT triplet spectra of p-ANBP 5.0 mM LiC1, filled circles; p-ANBP + 5.0 mM TBABr, empty circles; p-ANTP + 5.0 mM LiC1, thick line; and p-ANTP 5.0 mM TBABr. The spectra have been arbitrarily scaled.
+
+
TABLE 1: Final Positions of Spectral Maxima of the CT Triplet Absorption Bands of p-ANBP and p-ANTP in the Presence of Electrolyte@ sample
L,(nm)
Av (cm-I)
p-ANBP in neat THF p-ANTP in neat THF p-ANBP TBABr p-ANBP LiCl p-ANTP TBABr p-ANTP LiCl
710 990 610 490 730 570
2310 z t 100 6320 100 3600 i 100 7440 & 100
+ + +
Figure 4. Dependence of the triplet decay profiles of p-ANTP on the TBABr concentration. (a, top) Measured at 990 nm corresponding to the A,,,,, in neat THF: a, neat THF; b, 0.5 mM TBABr; c, 1.0 mM TBABr; d, 2.0 mM TBABr; e. 25 mM TBABr. (b, bottom) Measured at 730 nm corresponding to the I.,,, of the fully associated form: a, neat THF; b, 0.5 mM TBABr; c, 1.OmM TBABr; d, 2.0 mM TBABr; e. 4.0 mM TBABr; f, 10.0 mM TBABr; g, 25.0 mM TBABr. 7.OEt7
-
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.-
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The solutions contain 0.1 mM of p-ANBP or p-ANTP and 5.0 mM of a salt in THF.
that there is no appreciable preassociation of the ground state of the probe molecule with the counterions. The dynamics of the blue shift, and hence of the ion-pairing process, can be conveniently followed for a broad range of electrolyte concentrations. The CT triplet decay profiles in the presence of an electrolyte take on a biexponential appearance, with a fast component corresponding to the rate of ion association and a slow one which corresponds to the actual lifetime of the charge-separated triplet state. Naturally, the ratio of the amplitudes of the fast and slow components is wavelengthdependent. In the long wavelength portion of the spectrum (close to the original ,I/ of the free, unassociated probe) the fast ion-pairing component appears as a decay (Figure 4a), while at shorter wavelengths (closer to the /Imax of the fully associated probe) it gives rise to an increase of absorbance (Figure 4b). Interestingly, the observed association rate appears to vary linearly with the overall concentration of the salt (Figure 5) and not with the square root of the concentration of the salt, as one might anticipate on the basis of the Kohlrausch law.I2 This result
1 0.0
4.0
2.0
6.0
8.0
10.0
TBABr Concentration [mMl
Figure 5. Rate of association between the charge-separated triplet of p-ANTP and the ions (obtained from the fast lunetic component in Figure 4) plotted as a function of the TBABr concentration. Note the approximately linear dependence.
is not truly surprising as both salts are very weakly dissociated in THF. Therefore, the most likely mechanism leading to association of an ion with the charge-separated species is not an encounter with a free ion (as depicted in the Scheme l), but rather an encounter with a n "undissociated salt molecule", Le., a contact or solvent-separated ion pair of the salt, followed by dissociation of the original ion pair and creation of a new one, e.g. D+-A-
+ (Li'Cl-)
-
(D+-A-,Li-)
+ (Cl-,DL-A-) + (Cl-,D--A-,Li-)
+ ...
This is in agreement with the previous observations in moderately polar media.' At present the authors prefer not to speculate
J. Phys. Chem., Vol. 99, No. 8,1995 2253
Letters which of the possible resulting species in the above scheme is the dominant one. It is expected that departures from the simple linear behavior will be observed both for very low ( 1 x lo-’ M) electrolyte concentrations. The pseudounimolecular association rate in the presence of tetrabutylammonium bromide extracted from Figure 5 is 7.7 x lo9 M-I s-l, indicating a nearly diffusion-controlled process. On the other hand, even though LiCl produces the largest spectral shift, the kinetics of ion pairing in the presence of this salt appears to be much slower than diffusion controlled (approximately 1 x IO9 M-I s-l), suggesting the existence of a considerable activation banier associated with this process. This result is in agreement with the higher activation energy for the dissociation of LiC1, and it finds support in some earlier studies.2 The activation energies of the ion-pairing process in the presence of various salts will be investigated in detail in forthcoming temperature dependence measurements. To summarize, we have demonstrated that transient charge transfer absorption bands can serve as sensitive probes of ion association energetics and dynamics. The long-lived triplet charge transfer states enable one to extend the spectroscopic study of ion-pairing dynamics to weakly polar media and to highly dilute (submillimolar) solutions of electrolytes. Very large dynamic spectral shifts approaching 1 eV have been observed for p-aminonitroterphenyl in the presence of electrolytes. The large spectral shifts and the corresponding reorganization energy support the notion that ion-pairing and ionic atmosphere effects can play an important role in controlling the energetics and the lifetime of charge-separated states. A full report of our systematic study of the ion size dependence of these effects is in preparation.
Acknowledgment. The support of this work by the Office of Basic Energy Sciences, Division of Chemical Science, United States Department of Energy, under Grant FG-05-92-ER143 10 is gratefully acknowledged. References and Notes (1) Huppert, D.; Ittah, V.; Kosower, E. M. Chem. Phys. Lett. 1989, 159, 267-275. (2) Chapman, C. F.; Maroncelli, M. J . Phys. Chem. 1991, 95, 909591 14. (3) Thompson, P. A.; Simon, J. D. J . Chem. Phys. 1992, 97, 47924799. (4) Piotrowiak, P.; Miller, J. R. J . Phys. Chem. 1993,97, 13052-13060. ( 5 ) Tiede, D. Private communication. (6) As a matter of clarification, we should note that from the strict point of view the process of interest (Scheme 1) is an association between the dipolar charge-separated species (a “giant dipole”) and an ion. Throughout this paper, for the sake of simplicity, we will refer to this process as ion pairing, with the full understanding that this term is truly justified only at contact or at short distances, i.e., when the separation between the ion and the appropriate end of the “giant dipole” is smaller than the length of the photoinduced dipole itself. (7) O’Connor, D. B.; Scott, G. W.; Tran, K.; Coulter, D. R.; Miskowski, V. M.; Stiegman, A. E.; Wnek, G. E. J . Chem. Phys. 1992, 97, 40184028. (8) Davydov, B. L.; Kotovshchikov, S. G.; Nefedov, V. A. Sov. J . Quantum. Electron. (Engl. Transl.) 1977, 7, 129. (9) Paddon-Row, M. N.; Oliver, A. M.; W a r ” , J. M.; Smit, K. J.; de Haas, M. P.; Oevering, H.; Verhoeven, J. W. J . Phys. Chem. 1988, 92, 6958. (10) Gugliameli, L. C.; Franco, R. M. An. Asoc. Quim. Argent. 1929, 17, 340. ( 1 1) This laboratory, unpublished results. (12) Kohlrausch, F. Z. Elektrochem. 1907, 13, 333. JP943 1402
