10391
J. Phys. Chem. 1994, 98, 10391-10396
ARTICLES Picosecond Dynamics of Cation-Macrocycle Interactions in the Excited State of an Intrinsic Fluorescence Probe: The Calcium Complex of 4-(N-Monoaza-15-crown-5)-4'-phenylstilbene P. Dumon, G. Jonusauskas: F. Dupuy, Ph. Pee, and C. Rullibre* Centre de Physique Moliculaire Optique et Hertzienne, UA du CNRS No. 283, Universitt de Bordeaux 1, 351 cours de la LibCration, 33405 Talence, France
J.-F. Ldtard and R. Lapouyade* Laboratoire de Photophysique et Photochimie Molhxdaire, UA du CNRS No. 348, Universitt de Bordeaux 1, 351 cours de la Libtration, 33405 Talence, France Received: June 8. 1994@
Sub-picosecond absorption spectroscopy is used to examine the dynamics of calcium-azacrown interactions in the excited state of 4-(N-monoaza- 15-crown-5)-4'-phenylstilbene,in butyronitrile. This intrinsic fluorescence probe leads (in 8 f 2 ps) to a new emitting state which is assigned to a twisted intramolecular charge transfer state. When the probe is compiexed with Ca2+, the kinetic evolution of the absorption and gain bands is interpreted by a reversible breaking (in 8 f 2 ps) of the nitrogen-calcium bond followed by the reversible formation (in 50 f 10 ps) of a solvent-separated cation-probe pair.
CHART 1
1. Introduction The advent of supramolecular chemistry has introduced a systematic approach to the design of chemosensors with selective binding of species (ions or neutral molecules) and fluorescence as the optimal monitor.' We recently reported the synthesis and a steady-state fluorescence study of stilbene crowns [4-(N-monoaza-15-crown-5)stilbenes,Chart 11, which are intrinsic fluorescence probes in which the amine nitrogen possesses, simultaneously, an electron-donor function for the complexed cation as well as for the chromophore.2 The great majority of the intrinsic fluoroionophores described until now have almost identical photophysical properties like their counterparts with a dimethylamino group instead of the macr~cycle.~ But upon complexation by alkali or alkali-earth metal ions, they show a hypsochromic shift and a hypochromic change of the absorption spectrum and a quenching of the fluorescence, whereas the emission spectrum is only slightly blue-shifted and the fluorescence lifetime almost u n ~ h a n g e d . ~We . ~ interpreted the similarity of this fluorescence to the emission of the uncomplexed probes as a fast breaking of the cation-nitrogen interaction bond in the excited state as a result of the reduced electron density on the nitrogen when the chromophore is excited.2 The slight blue shift is as expected for the proximity of the dipolar emitting state and cation. In order to support or disprove this assignment, we synthesized new polar stilbene crowns (D-A stilbene crown) with decreasing electron affinity of the acceptor part (DS-crown < DCS-crown) and the ease of the twisted intramolecular charge transfer (T1CT)-state formation lowered (PDS-crown < DScrown).* We also expected that an emission shift, upon ligand binding, would occur with the less polar probes allowing the monitoring of the association equilibrium by ratiometric mea-
* To whom correspondence should be addressed. +
DRED postdoctoral fellowship. On leave from Vilnius University.
ca Abstract published in Advance ACS Abstracts, September 15, 1994.
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DCS
DS
DS-Crown
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surements6 while we hoped the more polar fluoroionophore would photorelease the cation.' In this paper, we present the absorption and steady-state fluorescence of the three D-A stilbene crowns and a picosecond study of the less polar probe (PDS-crown), in butyronitrile. The rates of the nitrogen-calcium bond dissociation and of the
0 1994 American Chemical Society
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10392 J. Phys. Chem., Vol. 98, No. 41, 1994
TABLE 1: Photophysical Characteristics of the Free (L) and Fully Complexed (LM) Stilbene Crowns, in Acetonitrile, at Room Temperature DCS-crown DCS-crown/Ca2+ DS-crown DS-crown/Caz+ PDS-crown PDS-crown/Ca2+
392 330 356 308 372 332
4790
4.0
46 4,5
4380
4.4
41 41
4.7
4,6 4,6
3240
525 503 437 424 483 383 469
0.06 0.06 0.01 0.01 0.27 0.22
830 700 620
L for ligand and LM for metal cation (M) complexed by L.
solvation shell reorganization with shielding of the cation from the chromophore are resolved for the fiist time.
2. Experimental Section 2.1. Materials. The stilbene crowns were synthesized as reported earlier.2 Solvents used were of spectrometric grade from either SDS or Merck. Butyronitrile (Fluka) was successively stirred with active carbon, K2CO3, KMn04, and P2O5 and distilled after filtration. Alkali-metal and alkaline-earth metal perchlorates purchased from Janssen and Alfa were vacuum-dried for more than 24 h at 140 “C and kept anhydrous over P205 in a dessicator. 2.2. Sub-picosecond “Pump-Probe” Experiments. Our laser system employed a hybridly mode-locked dye laser (Coherent 702) which was synchronously pumped by the second harmonic of an actively mode-locked CW-pumped Nd3+:YAG laser (Coherent “Antares-76”). An active stabilizer (Coherent 7670 active amplitude and position stabilizer) improved the stability of “Antares” output. The dye laser operated with rhodamine 6G as an amplifier medium and pinacyanol chloride as a saturable absorber.* The dye laser output, centered at 600 nm (0.5 W CW, pulse duration 500 fs fwhm), was amplified by a dye amplifier (“Continuum PTA 60”) pumped by the frequency-doubled output (532 nm, 10 Hz, 70 ps, 25 mJ) of the Nd:YAG regenerative amplifier (“Continuum RGA 10”). The dye amplifier delivered pulses with an energy of 1.5 mJ at a 10 Hz repetition rate. Dye amplifier output pulse parameters were controlled with a home-made single-shot autocorrelator. The pulse duration was of the order of 0.4 ps. The dye laser pulse at 600 nm was frequency doubled (KDP crystal), and the available energy at 300 nm was in the range 60-20 pJ with roughly the same pulse duration (0.4 ps). The pulses at 300 and 600 nm were separated by a beam splitter. The UV pulse (300 nm) was sent on the sample in order to excite the molecules to be studied. The 600 nm pulse, after passing through an optical delay line (60 fs steps), was focused on a rotating quartz plate to generate a continuum of light which extended from 320 to 900 nm. This continuum was sent through the sample as the probe beam and analyzed by means of a spectrograph (Chromex 5001 S) and an intensified photodiode array (Princeton IRY 512 S/R) controlled by a computer. Measuring the spectral distribution of the continuum probe in the presence and absence of the W pulse in the sample allows determination of the transient absorption spectra of the sample. Changing the position of the optical delay line allows the observation of the time evolution of the transient spectra (spectral shape and absorbance as a function of time) on a picosecond time scale. The UV pump pulse and the probe pulse were linearly polarized. By means of polarizers, we adjusted to 54.7” (the “magic angle”) the angle between polarization directions of these beams to ensure kinetics free from reorientational effects.
/i
300
\
350
400
450
500
550
Wavelength [nm)
Figure 1. Absorption and emission spectra in acetonitrile of PDScrown (-) and its calcium complex (- - -) in acetonitrile. TABLE 2: Charge Density on the Nitrogen Atom of the Model Compounds (Chart 1) Estimated by CNDO/S Calculationu
so SdW
A d s o - SILE) Si(CT)
DCS
DS
PDS
-0.357 -0.276 0.081 -0.135
-0.369 -0.302 0.067 -0.164
-0.369 -0.319 0.050 -0.171
The geometry and the method are as described in detail in ref 13.
3. Results 3.1. Absorption and Steady-State Fluorescence. The fiist absorption band of 4-(dimethy1amino)stilbenes has a pronounced charge transfer ~haracter,~ so that cation binding by their crown analogues induces a blue shift of the absorption spectrum attributed to the interaction of the nitrogen lone pair with the complexed cation (Table 1 and Figure 1). The complexation constant with calcium perchlorate, in acetonitrile, increases along the series: DCS-crown < DS-crown < PDS-crown (Table l), in accordance with the charge density on the nitrogen atom (Table 2). On the other hand, in polar solvents, excitation of the fully complexed probes (LM) leads mainly to the fluorescence of the free ligand (Table 1) which is only slightly blue-shifted as already observed for similar probe^.^,^^ The excited state responsible for the emission will be called (L*/S/M). Nevertheless a new short-wavelength emission band, which is the mirror image of the absorption and therefore from the Franck-Condon excited state called (LM)*, appears when the charge transfer character of the probe in the excited state is the lowest (Table 2). For example, in CH3CN, where DCS-crown and DS-crown present only the emission from (L*/S/M), PDS-crown also shows fluorescence from (LM)* (Figure 1 and Table 1). Also for PDS-crown/Ca2+,Z(LM)*/Z(L*/S~) increases when the polarity of the solvent decreases (Figure 2 ) . Scheme 1 accounts for the observed results. (i) The new short-wavelength emission of PDS-crown/Ca2+ is very similar to the fluorescence of the protonated probe (PDS-
J. Phys. Chem., Vol. 98, No. 41, 1994 10393
Piocsecond Dynamics of Calcium- Azacrown Interactions Fluo. I [a.u.]
t
Wavelength lnm]
Figure 2. Fluorescence spectra of PDS-crown/Ca*+in (Clz, (- - -) BuCN, and (-) CH3CN.
- -)
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crowdCF$2OOHsLH') and of the parent compound without the donor group (PS),2 which means that it originates from the excited state (LM)* of the complex ligand as formed in the ground state (Franck-Condon excited state). (ii) The long-wavelength emission of PDS-crown/Ca2+,which is shifted by only 620 cm-' ( A Y ~ Y - ~from ' ) that of the free ligand, points to the near vanishing of the interaction between the cation and the nitrogen atom of the fluorophore. The cation is then substituted by an additional solvent molecule or an associate perchlorate anion changing from monodentate to bidentate. We call (L*/S/M) this excited state, with a new state of coordination, by analogy with the classical solvent-separated ion pairs in benzylic anion-cation pairs, for example." Note that for a given solvent (CH3CN), the blue shift increases along the series PDS-crown < DS-crown .c DCS-crown (Table 1) and can be correlated with the charge on the nitrogen in the
emitting state (DE or TICT) (Table 2). This is evidence that after breaking of the cation-nitrogen bond, the cation stays in the proximity of the nitrogen within the lifetime of the emitting state. (iii) The decrease of the intensity of the short-wavelength band ((LM*-state emission), in polar solvents, illustrates the interplay of two factors: (1) the decrease of the nitrogen charge density upon excitation (Table 2) and (2) the interaction of the cation with the solvent, which both lead to the (L*/S/M) state with a proportion depending on the polarity of the solvent molecule which is substituted to the cation-nitrogen bond. For PDS-crown/Ca2+ in BuCN, a plot of the ratio of the two fluorescence bands versus 1/T (K) led to a bell-shaped curve which showed, among other things, that an equilibrium between (LM)* and (L*/S/M) takes place at temperatures higher than 263 K.2 Shortly after we presented these results,2 direct evidence of photoejection of lithium and calcium ions from a crown ether merocyanine was provided by picosecond pump-probe spectroscopy in a report by Martin et al.12 But while the photoejection process measured to be 5 ps for Li+ and 20 ps for Ca2+ may represent the dissociation of the nitrogen-calcium cation bond, it was our opinion that the departure of the cation from the crown should be slower because the process requires extensive nuclear motions. To gain more information on the detailed photodissociation of the cation from the probe, we have undertaken a sub-picosecond kinetic investigation. 3.2. Sub-picosecond Spectroscopy. In order to see the effect of a close calcium cation, we recorded the time evolution of the transient spectra of the free fluoroionophore ([L] = 1 x
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SCHEME 1
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Dumon et al.
10394 J. Phys. Chem., Vol. 98, No. 41, 1994 PDS-Crown/Ca2'
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Figure 5. Wavenumber at half-height ( ~ 1 1 2 )of the red side of the M PDS-crown and absorption band as a function of time of (1) M PDS-crown, 5 mM Ca(C104)2, in BuCN, at 20 "C. (2)
lop4 mole L-l, Figures 3a and 4a) and of PDS-crown complexed with an excess of Ca2+ ([Mol = 5 x mole L-l) in BuCN (Figures 3b and 4b). 3.2.1. Free Fluoroionophore (PDS-Crown). The transient spectra display a broad absorption band extending from 5 10 to 750 nm (see Figure 3a) and two gain bands: one band peaking at 390 nm and another whose maximum moves from 440 to 465 nm (see Figure 4b). This last band will be called hereafter the 440-465 nm gain band. The shape of the absorption band changes as a function of time (see Figure 3a). The most evident point is a blue shift of the red part as well as some changes concerning its maximum and structure. The general behavior can be seen as the evolution of two slightly different bands which strongly overlap, the band appearing at short time having a more important contribution on the red side of the transient spectra. By plotting the evolution of the wavenumber at the half-height ( ~ 1 1 2 )on the red side of this band (see Figure 5, curve l), we fitted this decay by the main contribution (95%) of a 7 f 2 ps exponential decay and a small contribution (5%) of a 100 f 20 ps exponential decay. This last contribution is inside the experimental error and difficult to ascertain. From these results, it is clear that two
excited species exist and evolve after excitation in a precursorsuccessor relationship with a conversion time of 7 f 2 ps. The two gain bands with negative optical densities peak at 390 and 440-465 nm. Because of its spectral range near the spontaneous emission spectrum (A, = 478 nm)? we can assign the 440-465 nm band to an amplification of the probe beam, by stimulated emission from excited states. We plotted the maximum wavelength of the 440-465 nm gain band and measured the shift to the red as a function of time. This shift decays monoexponentially with a constant time of 7 f 2 ps (see insert in Figure 4a); the same value was obtained for the shift of the red part side of the absorption band. Assignment of the 390 nm band is more difficult. Fluorescence emission is very weak in this spectral range (see Figure l), and therefore this band cannot be assigned to stimulated emission as for the 440-465 nm band. Moreover ground-state absorption is very strong in this spectral range, with a maximum at 374 nm.* So it is quite reasonable to assign this gain band to a bleaching of the ground state. Bleaching is not observed for A < 330 nm because of the spectral sensitivity of the setup limited by the spectral range of the probe beam, which falls off at 330 nm. From these results, we can also assert that two excited species are emitting but with large overlapping of the emission spectra. In order to get additional evidence for the existence of these two excited states, we also plotted the variation of the optical densities as a function of time for different spectral ranges. The results are shown in Figure 6. At any part of the spectra, we observed a biexponential decay with lifetimes of 8 f 2 and 500 f 100 ps. Only the respective contributions of the two exponential decays depend on the considered spectral range (see Figure 6), showing the large overlap of the spectra of the excited states. 3.2.2. Complexed PDS-Crown with Ca2+. As for PDS-crown, we observed a large absorption band in the range 510-750 nm and gain bands in the range 360-460 nm (Figures 3b and 4b), which spectrally evolved after excitation. The absorption band shows a pronounced blue shift of the red side (see Figure 3b) and at longer times becomes similar to the absorption band of the free probe (1- = 620 nm, see Figure 3a). As for PDS-crown, we plotted the wavenumber at halfheight on the red side ( V I / $ of the absorption band. The results
Piocsecond Dynamics of Calcium- Azacrown Interactions
J. Phys. Chem., Vol. 98, No. 41, 1994 10395
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Figure 6. Intensity kinetics of transient spectra integrated over different M PDS-crown in butyronitrile. (1) Absorption spectral ranges for band from 532 to 659 nm. (2) Absorption band from 659 to 652 nm. (3) Gain band from 436 to 484 nm. (4) Gain band from 365 to 413
nm. The full lines are calculated curves according to kinetic Scheme 2 with k12 = (1.3 f 0.3) x 10" sWL and kl = k2 = (2.0 f 0.4) x lo9 s-l and assuming a Gaussian excitation pulse with half-duration of 0.4 PS. are shown in Figure 5 (curve 2) and illustrate the effect of calcium on the shift of the PDS-crown transient absorption. The kinetic of the shift is clearly biexponential with equal contributions (respectively 60% and 40%) from a short decay (8 f 2 ps) and a long decay (50 f 5 ps). Furthermore we may observe that the total amplitude of the shift is larger (1500 cm-') in the presence of calcium that without calcium (500 cm-'). Also, we have to note that at longer times (> 200 ps), v1/2 reaches the same value in the presence and absence of calcium, showing that the two excited samples reach the same state. The influence of the calcium cation is also well illustrated in the spectral range 360-460 nm. Just after excitation, a gain band first appears at 370 nm followed, delayed in time, by an additional band around 450 nm as shown in Figure 4b. Spectral changes of the 450 nm band have been observed, mainly a red shift of the wavelength of its maximum. The 370 nm band can be assigned to the complexed excited species ((LM)") for the following reasons: (1) It corresponds to the emission spectra of this species (see Figure I). ( 2 ) It appears immediately with the excitation. (3) The ground-state absorption spectrum, in the presence of calcium, is shifted to the blue (see Figure 1) and bleaching cannot be involved in this spectral range just after excitation. The influence of calcium is also well illustrated by the optical density variation of the absorption and gain bands as a function of time. These variations are illustrated in Figure 7 for different spectral ranges. For PDS-crown/Ca*+,the intensity kinetics are more complex than for PDSkrown and an intermediate component appears on the decays which are triexponential (see, for comparison, Figures 6 and 7). From these different results (triexponential decay of the absorption and emission band intensities, spectral evolution of the absorption band with the presence of two decay components (see Figure 5 ) , gain bands appearing with different rise times), it is obvious that addition of calcium creates new evolution channels and, as a consequence, new excited states with respect to the free probe. Let us now discuss these results and give the possible explanations. 4. Discussion
The biexponential decay of the free probe (PDS-crown) shown in Figure 6 associated with the spectral evolution of the
0
200
100
300
400
500
TIME IN P S Figure 7. Intensity kinetics of transient spectra integrated over different spectral ranges for 5 mM Ca(C104)z in BuCN. (1) Absorption band from 707 to 715 nm. (2) Gain band from 370 to 475 nm. The full lines are calculated curves according to kinetic Scheme 3 with klz = (1.3 f 0.3) x 10" s-', k21 = (6.6 f 1.6) x 10" S-I, k23 = (2.0 f 0.4) x loLos-', k32 = (6.9 f 1.4) x lo9 s-', and kl = kt = k3 = (2.0 i 0.4) x lo9 s-I and assuming a Gaussian excitation pulse with halfduration of 0.4 ps.
SCHEME 2 hv
SO
$2
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I
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lk2
transient spectra is the first direct evidence for two excited and fluorescing states in polar stilbenes, at low excitation intensity.14 In previous studies, we observed a lengthening of the singlet lifetime of the DS derviatives, when the temperature is increased from 77 to 172 K for PDS-crown, that we explained by a rotational relaxation leading from the Franck-Condon state (DE) to a twisted intramolecular charge transfer state.2 Comparison of the photophysical properties of regioselectively bridged DS derivatives locates the decoupling site at the single bond between the electron-donor group (N,N-dimethylaniline) and the electron-acceptor moiety (arylethylene). From the solvatochromism of the fluorescence, we calculated a dipole moment of 15.8 D in the excited state for PDS-crown2 which corresponds to a large charge transfer. It is then resonable to assume the formation of a TICT state in this compound. According to this discussion, the model shown in Scheme 2 should account for the experimental observations under excitation. Excitation produces the DE state. The emission at 440 nm (appearing just after excitation) and the wide and structured absorption band can be attributed to this state. From this Franck-Condon excited state, charge transfer occurs leading to the formation of the CT state which has to be stabilized, the stabilizationprocess involving solvent relaxation as well as some structural changes. The observed spectral evolution of the emission band at 440 nm (red shift of 800 cm-') as well as the blue shift of the transient absorption band to a structureless band is the illustration of this stabilization process accompanying the formation of this state. We propose that the unresolved absorption band and the shifted gain band near 460 nm correspond to the TICT state. However the fact that the spectra of these different species are not markedly different indicates that the amplitude of the structural changes does not lead to a complete uncoupling of the electronic donor-acceptor moieties as already discussed with D-A ~ti1benes.I~We fitted the results
Dumon et al.
10396 J. Phys. Chem., Vol. 98, No. 41, 1994 SCHEME 3
-
(LM)*
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k32
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of Figure 6 according to Scheme 2 . In the fitting procedure, we took into account the pulse excitation which was introduced in the model as a Gaussian-shaped pulse of 0.4 ps half-width. This value was obtained by measuring the autocorrelation function of this pulse. The results of the fit are illustrated in Figure 6, showing that the proposed model is quite reasonable. During the fitting procedure, we tried to introduce a possible equilibrium between the DE and TICT states, but the fit was not satisfactory. Only for the given kinetic constants (see the caption of Figure 6) was the fit acceptable. Taking into account that the calculated 8 4z 2 ps value for the formation rate of TICT state was also directly deduced from the spectral shift of absorption and emission bands tends to confirm that the rate constant of formation is quite near this value and that the model is quite consistent with the different experimental observations. The description of the photophysical behavior of PDS-crown was a prerequisite to understand the effect of the cation in a parallel study with the calcium complex of PDS-crown. Let us now discuss the influence of the calcium, having in mind that the TICT state can be formed in PDS-crown. Compared with the observations described above, when calcium is added to a solution of PDS-crown, obviously it is necessary to introduce a third state which should be related to a third relaxation channel, whose presence is illustrated by the kinetics shown in Figures 5 and 7. Incidentally we have to note that we tried unsuccessfully to fit Scheme 2 to the results shown in Figure 7. This failure led us to build a different scheme. We propose and then discuss Scheme 3 to explain these results. This model fits the kinetic curves of Figure 7 quite well, as can be seen, with the parameters given in the caption of Figure 7. However, with regard to the number of parameters and the experimental precision, these fits are not absolute proof of the validity of the model but only an indication that it can work and be reasonable. We have to note, for example, that it is possible to fit these curves without assuming equilibrium between the different species. But we have observed, from steady-state measurements, that equilibrium exists.2 So it was necessary to introduce this equilibrium in the model to take into account all the available experimental results. Under these circumstances, we found the rates given in the legend of Figure 7. Scheme 3 can then be rationalized as follows: Excitation of the PDS-crown/Ca2+ complex leads to the excited state (LM)* of the fluorophore without noticeable charge transfer because the bound cation lowers the electron-donating character of the nitrogen atom. The (LM)* state gives the gain band at 370 nm (Amm = 387 nm for the steady-state emission) and the main absorption band in the long-wavelength region (510-750 nm). The instantaneous ( e1 ps) rise time (excitation pulse duration) of these bands supports this assignment. From (LM)* where the charge density on the nitrogen has been lowered, the reversible dissociation of the nitrogen-cation bond occurs in 8 f 2 PS (k12 = (1.3 k 0.3) x 10" s-'; k21 = (6.6 f 1.6) x 10" s-l), leading to the cation-probe contact pair (L*M) which reversibly (k23 = (2.0 f 0.4) x 1O1O s-l; k32 = (6.9 & 1.4) x lo9 s-l) leads to a solvent-separated cation-probe pair, (L*/ S/M), or to one perchlorate anion becoming bidentate to satisfy
the coordination sphere of Ca2+ in 50 f 10 ps. Because of these fast equilibria, the decay to the ground state appears monoexponential with k3 = ( 2 0.5) x lo9 s-l. The spectral shifts shown in Figure 5 also support this model. The ~ 1 1 value 2 at earlier times is higher in PDS-crown/Ca2+than in PDS-crown. Regarding the less pronounced charge transfer character of the (LM)* state, this is an expected behavior. When the nitrogen-cation bond is broken, the charge transfer occurs concomitant with a large stabilization energy. The 1500 cm-' amplitude of the ~ 1 1 2shift, in PDS-crown/Ca2+,to be compared to 500 cm-I observed in PDS-crown, is then quite consistent with this model. From these results, it is difficult however to rigorously measure the formation of the TICT state in the complexed probe. Nevertheless this formation process necessarily follows the transfer of charge that is reflected by the blue shift of the transient absorption (Figure 6) as well as the red shift of the gain band at 460 nm. It is then obvious that the TICT state formation is slowed down by the proximity of the cation. We must note that by measuring the transient evolution of the transfer of charge in the probe we have been able to infer the coordination state of the proximate cation. We believe that it is a general property of intrinsic fluoroionophoresthat is worth pursuing. Such work is in progress.
*
5. Conclusion From a TICT-forming fluoroionophore (stilbene crown), we directly observed and measured, for the first time, the rate constants of the reversible nitrogen-calcium bond dissociation in the excited state. The value obtained, in butyronitrile (kd, = (1.3 f 0.3 x s-'), compares well with the rate measured by Martin et al.I2 in a related system (DCM-crown), in acetonitrile. But besides this, we have also observed and measured for the first time the rate constants for the reversible formation of what we think is a solvent-separated cation-probe pair.
Acknowledgment. This work was made at the Centre National Impulsions Laser Ultra-Courtes, financially supported by DRED (Direction des Recherches et Etudes Doctorales, Ministere de 1'Education) which is gratefully acknowledged. References and Notes (1) Fluorescent Chemosensors for ion and molecule recognition; Czamik, A. W., Ed.; American Chemical Society: Washington, DC, 1992.
(2) LCtard, J.-F,; Lapouyade, R.; Rettig, W. International Symposium on Experimental and Theoretical Aspects of Excited State Electron Transfer and Related Phenomena, Pultusk, Pologne, Sept. 27-Oct. 2, 1992. Ibid. Pure Appl. Chem. 1993, 65, 1705. (3) LCtard, J.-F.;Lapouyade, R.; Rettig, W. J . Am. Chem. SOC.1993, 115, 2241. (4) Bourson, J.; Valeur, B. J . Phys. Chem. 1989, 93, 3871. (5) Rettig, W.; Lapouyade, R. Topics in Fluorescence Spectroscopy N. Probe and Chemical sensing; Lakowicz, J. R., Ed.; Plenum Press, New York, 1994. (6) Tsien, R. Y. Annu. Rev. Neurosci. 1989, 12, 227. (7) Adams, S. R.; Kao, J. P. Y . ;Grynkiewicz, G.; Minta, A,; Tsien, R. Y. J . Am. Chem. SOC. 1988, 110, 3212. (8) HCbert, P.; Marguet, S.; Gustavsson, T.; Mialocq, I. C. Opt. Commun. 1992, 90, 85. (9) Fery-Forgues, S.; Le Bris, N. T.; Guette, J.-P.; Valeur, B. J . Phys. Chem. 1988, 92, 6233. (10) Jonker, S . A.; Ariese, A,; Verhoeven, J. W. Recl. Trav. Chim. PaysBas 1989, 108, 109. (11) Hogen-Esch, T. E.; Smid, J. J . Am. Chem. SOC. 1965, 87, 66. (12) Martin, M. M.; Plaza, P.; Dai Hung, N.; Meyer, Y. H.; Bourson, J.; Valeur, B. Chem. Phys. Lett. 1993, 202, 425. (13) Mtard, J.-F.; Lapouyade, R.; Rettig, W. Chem. Phys. 1994, 186, 119. (14) Gilabert, E.; Lapouyade, R.; Rullibre, C . Chem. Phys. Lett. 1991, 185,82.
