15709
J. Phys. Chem. 1995,99, 15709-15713
Picosecond Transient Absorption as Monitor of the Stepwise Cation-Macrocycle Decoordination in the Excited Singlet State of 4-(N-Monoaza-15-crow-5)-4'-cyanostilbene~ Renaud Mathevet, Gediminas Jonusauskas, and Claude Rulli&re* Centre de Physique Molbculaire Optique et Hertzienne (CPMOH) U.A. CNRS No. 283, Universitb de Bordeaux I, 351 cours de la Libtration, 33405 Talence Cedex, France
Jean-Franqois LCtard and RenC Lapouyade" Laboratoire de Sciences Molbculaires, Institut de Chimie de la Matiere Condensbe de Bordeaux (ICMCB), Chateau Brivatac, Avenue du Dr A. Schweitzer, 33608 Pessac Cedex, France Received: May 12, 1995; In Final Form: September 14, 1 9 9 9
Picosecond absorption spectroscopy of 4-(N-monoaza- 15-crown-5)-4'-cyano stilbene, a polar fluoroionophore of the stilbene-crown series, shows the formation of an intramolecular charge-transfer state (TICT state), in less than 1 ps (k > 10l2 s-')in acetonitrile (CH3CN) and less than 4 ps (k = (2.5 f 0.7) x 10" s-l) in butyronitrile (BuCN). When this fluoroionophore is fully engaged in a calcium complex, its electronic excitation leads sequentially from the Franck-Condon state (LM)*, to a cation probe pair (L*M) (k = (2.5 f 0.5) x 10" s-l in CH3CN and k = (3.3 f 0.5) x 10" s-l in BuCN) and to a solvent separated cation probe pair (L*/S/M) (k = (1.4 f 0.5) x 1O'O s-l in CH3CN and k = (4 f 0.5) x 1O'O s-l in BuCN). The transient absorption spectra of these three intermediates on the decoordination pathway have been obtained. The increasing charge transfer of the three states is controlled by the ligand exchange in the coordination sphere of the calcium cation.
Introduction The effects of salts on chemical reactions both in the ground' and the excited2 electronic states have been recently reviewed. Their understanding should benefit from the renewed interest in the role of electrolytes and counterions in intramolecular charge-transfer reaction^.^ Most of the significant results came from the studies of time-dependent fluorescence shifts of suitable probe molecule^.^^^ Molecules that upon electronic excitation strongly modify their electronic distribution are very attractive in this respect. They are also extensively investigated because of their nonlinear optical susceptibilities.6 Within this series we are currently studying the photophysics of polar diphenylpolyenes having both electron donor and electron acceptor groups. We showed that chemical modifications of one of these substituents leads to chromoionophores able to transform chemical information (produced by an ionophore-ion interaction) into an optical signal (fluorescence emi~sion).~ Fluorescence is for many reasons the optimal signal transduction mechanism in sensing applications:8 It may be indeed characterized by its intensity, by the intensity ratio of different spectral components, and by its lifetime. Furthermore it is a very sensitive probe since even the fluorescence of a single molecule can be o b ~ e r v e d . ~ . ' ~ We showed that the use of intrinsic fluoroionophores, Le., chemosensors in which the analyte interacts with a ligand that is part of the fluorophore n-systems,11v'2could lead to a blueshifted emission with ligand binding7 and to a red-shifted emission when the cation is released upon electronic excitation of the complexed probe.I3 Chemical changes upon excitation (the cation release) are then well correlated with intrinsic fluorescence properties in these systems. RecentlyI3 we investigated the dynamics of the calciumazacrown interaction in the excited state of 4-(N-monoaza- 15-
* To whom correspondence @
should be addressed. Dedicated to Prof. Hans-Dieter SCHARF on his 65th birthday. Abstract published in Aduance ACS Absrrucrs, October 15, 1995.
crown-5)-4'-phenylstilbene (PDS-crown). Picosecond absorption spectroscopy allowed us to measure the rates of the successive steps of nitrogen-calcium decoordination and solvent separated cation probe pair formation.I3 However, the chargetransfer character of PDS-crown proved to be insufficient to bring about the spectral evolution necessary to deconvolute the spectra of the different intermediates. In view of the usefulness of a better understanding of the cation-modulated photoinduced charge-transfer processes, we decided to investigate 4-(N-monoaza-15-crown-5)-4'-cyanostilbene @CS-crown) which has a stronger charge-transfer character. The strength of the charge-transfer character of this probe, combined with the polarity of the solvent, should induce large spectral changes, allowing one to distinguish the absorption spectra of the transient states. In this study we present the kinetic behavior of the calcium complex of DCS-crown in two solvents of different macroscopic polarity (butyronitrile (BuCN) and acetonitrile (CH3CN)) and compare the results with those already obtained with PDS-crown.
DCS-crow
PDS.Crom
Experimental Section Materials. The stilbene-crowns were synthesised as reported earlier? Acetonitrile was of spectrometric grade from SDS. Butyronitrile from Fluka was successively stirred with active carbon, K2C03, KMnO4, 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 P2O5 in a desiccator. Picosecond "Pump-Probe" Experiments. The experimental set-up has been described in detail e1~ewhere.I~The laser system delivers a typical output pulse of m l ps duration,
0022-3654/95/2099-15709$09.00/0 0 1995 American Chemical Society
15710 J. Phys. Chem., Vol. 99, No. 43, 1995 centered at 600 nm with an energy of 1.5 mJ at a 10 Hz repetition rate. The second harmonic of this pulse at 300 nm is used as a pump pulse to excite the molecules to be studied. The 600 nm pulse, after passing through an optical delay line (60 fs steps), is focused on a rotating quartz plate to generate a continuum of light which extends from 330 to 780 nm. This continuum is sent through the sample as the probe beam. Changing the position of the optical delay line allows observation of the transient spectra (spectral shape and optical density as a function of time) on a picosecond time scale. The angle between the polarizations of the probe and pump beams were adjusted to 54" (the "magic angle") to ensure kinetics free from reorientational effects. Correction and Fitting of the Results. Chirp Correction of the Transient Spectra. Because of group velocity dispersion (GVD) and self-phase modulation (SPM), the spectral components of the probe beam do not all reach the sample at the same time. At short delays this effect, known as spectral "chirp", affects the kinetics and induces spectral deformation of the recorded spectra. Using the optical Kerr effect in pure solvent, we measured this chirp from 350 to 780 nm. All the spectra presented are corrected for this effect using a homemade algorithm. Fitting and Modeling the Kinetics. Modeling was based on the assumption that after excitation the population of excited molecules formed in a first excited state (the Franck-Condon state) can be transformed into successive excited states until it reaches a final state which decays to the ground state. If the number of possible excited states is N , then the kinetic scheme can be described as follows:
- -s, -s, -so
sohv s,
k,2
k 2 ~
k~
where the Si are the different excited states involved in the transformation process and the kv are the kinetic constants. Each Si state has its own characteristic absorption spectra, with the molar absorption coefficient €;(A) at wavelength A. If at time tp the population of excited state Si is Pi(tp),the optical density OD (tp,A,) at time tp and wavelength A,, is then:
In eq 1, &,) strictly should be also a function of time tp, since solvent relaxation should induce a time-dependent shift of the spectra. However, as an initial approximation, we neglect this process for two reasons: (1) the observed spectral changes are too strong to be accounted only by a shift due to solvent relaxation; (2) the precision of the measurements is not sufficient to allow introducing, in the deconvolution procedure, one another parameter taking into account this process. Inside this approximation and from the kinetic scheme described above, the Pi populations should follow the differential equation:
From the measured optical density at different wavelengths in the spectral range 330-780 nm, the system of eqs 1 and 2 leads to the different kinetic constants kv and the absorption spectra ~i(1) of the states involved. The precision of the measurements limited the number of states to a maximum of three ( N = 3). In the fitting procedure the excitation pulse was introduced in the
Letters
300
400
HI0
600
Wavelength (nm)
'
400
'
560
'
660
'
700 ' 400 500 Wavelength (nm)
600
700
Figure 1. (a) Steady-state absorption and emission spectra of DCScrown. Full line: without Ca2''. Dotted line: with Ca 2+. (b) Transient spectra at different times after excitation. (A) DCS-crown in BuCN. (B) DCS-crown/Ca2+in BuCN.
model as a gaussian shaped pulse whose half-width was measured by optical Kerr effect.
Results Absorption and Steady-State Fluorescence. DCS-crown forms a ground-state complex with calcium perchlorate in acetonitrile but with a lower association constant (log Ks = 4) than PDS-crown (log Ks = 4.7). In butyronitrile these association constants are slightly higher. In the resulting complexes the lone pair of the nitrogen atom is in an electrostatic stabilizing interaction with the cation. As a result the first ground-state absorption band, which has a strong charge-transfer character in the free chromophore, is blue shifted7in the complex (Figure la). For emission, in both solvents (BuCN or CH3CN), DCScrown presents apparently only one fluorescence band, slightly blue shifted compared to the emission of the free chromophore without calcium. We showed previouslyI3 that the emission spectrum from the calcium-complex of the less polar probe, PDS-crown, leads to an additional short-wavelength band, mirror image of the absorption band of the complex. The presence or the absence of the emission band due to the complex is controlled by the competition between the emission process and the cation-nitrogen atom decoordination. The absence of the emission of the complex in DCS-crown has been assigned to an increased charge transfer character, compared to PDS-crown, and therefore a larger decrease of the nitrogen charge density upon excitation.I3 Picosecond Spectroscopy. Hoping to better characterize the different species along the decoordination pathway and to measure the corresponding rate constants, we recorded the time evolution of the transient spectra of the free fluoroionophore (DCS-crown, 1 x M) and of the complex, DCS-crown-
J. Phys. Chem., Vol. 99, No. 43, I995 15711
Letters
From these different results it is obvious that addition of calcium creates new evolution channels and, as a consequence, new excited states compared to the free probe. Let us now to discuss these results and to give the possible explanations. Discussion
1.5 1.o
0.5 0.0
Kinetic scheme
Figure 2. Results obtained according to procedure described in the text for the free probe DCS-crown. (A) and (B) are the spectra of the states SI and Sz in BuCN and CH3CN respectively and postulated according to the kinetic model presented in (D) and Scheme 1. (C) shows the population kinetics experimentally measured and simulated according to model (D) and Scheme 1. The values of the rate constants are presented in Table 1.
Ca2+, (concentration of Ca(C104)2 = 5 x M), in BuCN and CH3CN (Figure lb). Free Fluoroionophore (DCS-crown). The behavior of the transient spectra are very similar in CH3CN and BuCN, only the kinetic is slightly different. Following excitation the transient spectra display a broad absorption band with a maximum near 540 nm in BuCN (520 nm in CH3CN; see Figure lb). As time passes this broad absorption band tums into a structured band, the maximum shifts to the blue and a gain band appears between 500 and 600 nm, peaking near 570 nm in BuCN (580 nm in CH3CN). Analysis of these time-resolved spectra, using the mathematical procedure described above, show that two excited states SI and S 2 have to be involved to explain this behaviour, according to the following model:
so-hv s,
k12
s,
+
k20
s,
+
Our analysis allowed to reconstruct the absorption spectra of the SI and SZexcited states and to simulate the kinetics. These spectra and kinetics are shown in Figures 2. All these results are consistent with a precursor-successor relationship between the states S I and S2. We observed that the formation rate k12 of SZ from S I was faster in C H F N (k12 > 10l2 s-l) than in BuCN (kl2 = (2.5 k 0.7) x 10" s-l). DCS-crown-Ca2+ Complex. The time-resolved absorption spectra of the DCS-crown-Ca2+ complex in CH3CN and BuCN are shown in Figure lb. Within the pulse duration, a broad absorption band with a maximum at 580 nm and a small gain band near 425 nm appear. While the gain band disappears in x 3 ps, the long-wavelength maximum of the absorption band shifts to the blue (520 nm) and a new maximum appears at 450 nm. To account for the dynamic behavior we now need three distincts species, one more than required for the analysis of the free probe, according to the following scheme:
so-hv
SI
-s, -s, k12
k23
k30
so
+
The calculated spectra of the three states are shown in Figure 3 as well as the kinetics. We may observe that the results of the fits of the kinetics are quite good, giving confidence in the model.
We have recently thoroughly investigated the photophysics of "push-pull" stilbene derivatives (including DCS). We observed the evolution of the fluorescence lifetime as a function of the temperature. The results were rationalized within a three excited-state kinetic scheme including (1) the Franck-Condon state (DE for delocalized excitation) assumed to be planar, polar, and fluorescent), (2) a TICT state (twisted anilino group, highly polar, fluorescent), and (3) a so-called P* state where the double bond is twisted and which is not flu0re~cent.I~ But the first direct evidence for two successive excited states (DE and TICT) came from our recent report on the picosecond absorption spectroscopy of P D S - c r o ~ n . ' ~In BuCN, PDS-crown was shown to lead from the Franck-Condon state to a TICT state in 8 ps ( k = 1.3 x 10" s-I). DCS-crown presents similar photophysical behavior, particularly a lengthening of the fluorescence lifetime when the temperature is increased from 77 to 175 K that we assigned to a rotational relaxation leading from the Franck-Condon state (DE) to a twisted intramolecular charge transfer (TICT) state.7 The successor-precursor relation observed in DCS-crown and the evolution of the transient spectra are in favor of a DE to TICT state transformation, also accounted for by the following model:
SCHEME 1 SO
hv
DE
TXCT
With the polar probe DCS-crown, excitation produces the DE state (with corresponding spectra SIof Figure 2). Charge transfer occurs from this Franck-Condon excited state, leading to the formation of the TICT state (with spectrum SZ in Figure 2) by a stabilization process involving solvent relaxation as well as some structural changes. The faster rate constant k12observed in CH3CN compared to BuCN (see Table 1) is well correlated with the polarity of the solvent, as is expected for a chargetransfer process. The observed blue shift of the wavenumber vmaXof the maximum of the transient absorption spectra (see Figures l b and 4) illustrates this stabilization process accompanying the formation of this state. As shown in Figure 4, the stabilization process involves an energy of 1200 cm-I for example in BuCN to be compared, in the same solvent, to 750 cm-' in P D S - c r 0 ~ n . l ~Also, we were able to fit the kinetics of the blue shift with the rate values measured or calculated above (see Table 1) and the results are quite good (see Figure 4). These correlations confirm that the model proposed is quite consistent with the different experimental observations. For the first time the transient absorption of the DE and TICT states of D-A stilbenes are thus measured. Altough there is some overlap between the absorption and gain bands, the absorption spectrum of the TICT state peaks around 500 nm, in the region of the dialkyl aniline radical cation absorption.I5 This supports the assignement to a TICT state with the phenyl-azacrown as the electron donor group. With PDS-crown the transient absorption was to longer wavelengthsI3 certainly pointing to a stronger coupling with the DE state.I6 The formation of an emissive intramolecular charge-transfer state on the same time
Letters
15712 J. Phys. Chem., Vol. 99, No. 43, 1995
005
2
h
ow
O
I
0
*
0.1'
L
'
760
W l a n S m ( nm)
Kinetics&"
Figure 3. Results obtained according to procedure described in the text for the DCS-crowdCa2+.(A) and (B) are the corresponding spectra of the S I , S2, and S3 states respectively in CHKN and BuCN and postulated according to the kinetic model presented in (D) and Scheme 2. (C) are the population kinetics simulated and experimentally measured according to model (D) and Scheme 2. The values of the rate constants are presented in Table 1.
20.5"".'.'"
'
I
'
I
0 255075~b3200300400
4005(X36aI700
way ( PSI
Wdength (m)
Figure 4. (A): Shift as a function of time of the wavenumber of the maximum Y,,, of the transient absorption spectra for DCS-crown with and without calcium in BuCN. Full lines are the simulated curves according to the formula ( A exp(-t/tl) B exp(-r/t2)) with t1 = Uk12 and t2 = l / k ~(k12 and k23 are given in Table 1). (B) Experimental (squares) and calculated (full line) spectra of DCS-crowdCa2+in BuCN ( t > 100 ps). The calculated spectrum is a linear combination of spectrum Sz of Figure 2 (A) and spectrum S2 of Figure 3 (B) (see text
+
for explanation). scale (2-3 ps in acetonitrile) has also recently been reported for the merocyanine dye DCM.I7 The description of the photophysical behavior of DCS-crown was a prerequisite to understanding the effect of the cation in a parallel study with the calcium complex of DCS-crown. Let us now discuss the influence of calcium, having in mind that the TICT state can be formed in DCS-crown. We tried unsuccessfully to fit Scheme 1 to the results shown in Figure 3. This failure led us to build a different scheme. Following the scheme previously used to analyse the results obtained with the calcium complex of PDS-crown,13we had to introduce a third state leading to Scheme 2. SCHEME 2
(LM)*
kl2
(L*M)
k23
(L*/S/M)
1
k30
This model fits the kinetic curves of Figure 3 quite well, as can be seen, with the parameters given in the Table 1. However, with regard to the number of parameters and the experimental precision (f5%), these fits are not alone absolute proof of the validity of the model but they give it credibility and quantify a reasonable pathway. Scheme 2 can be rationalized as follows: Excitation of the DCS-crowdCa2+ complex (LM) leads to the excited state of the fluorophore (LM)* without noticeable charge transfer because the bound cation lowers the electron-donating character of the nitrogen atom. The (LM)* state gives the small gain band at Amax = 425 nm and has the absorption shown as spectrum SIin Figure 3. The instantaneous rise time (within the excitation pulse duration) of this absorption spectrum supports this assignment. From (LM)* where the charge density on the nitrogen has been lowered the dissociation of the nitrogen-cation bond occurs in 3 f 1 ps [ ~ I Z= (3.3 f 1) x 10" s-l] in BuCN and in 4 & 1 ps [k12 = (2.5 f 0.5) x 10" s-'1 in CH3CN ,leading to the cation-probe contact pair (L*M) with absorption spectrum SZ (see Figure 3) and a large chargetransfer character. We may observe that this spectrum is quite similar to the spectrum of the DE state observed in free DCScrown (spectrum S I in Figure 2 ) . This shows that the amplitude of the charge transfer, as expected after breaking the nitrogencalcium bond, nearly recovers the amplitude of the free probe in the Franck-Condon excited state. After this step, a further blue-shift of the transient absorption spectrum, indicating an increase of the charge-transfer character, is assigned to a change of the (L*M) state into a solventseparated cation probe pair, (L*/S/M with spectrum S3 in Figure 3), to satisfy the coordination sphere of Ca2+. The rate constant of formation is respectively k23 = (4 f 0.4) x 1Olos-l in BuCN and k23 = (1.4 f 0.4) x 1Olos-l in CH3CN. Then decay to the ground state occurs with k30 of the order of 2 x lo9 s-l (see Table 1). The shifts of the maximum of absorption spectra shown in Figure 4 also support this model. The value of Ymax at short times is smaller in DCS-crown/Ca2+ complex (16 500 cm-' in CH3CN and 17 500 cm-I in'BuCN) than in DCS-crown (19 300 cm-I in CH3CN and 18 700 cm-I in BuCN). This behavior is expected because of the less-pronounced charge-transfer character of the (LM)* state. When the nitrogen-cation bond is broken, the charge transfer occurs concomitantly with a large stabilization energy. For example in BuCN the 2970 cm-' amplitude of the vmax shift, in DCS-crown/Ca2+, has to be compared to 1100 cm-I observed in DCS-crown. It is then quite consistent with this model. Also, the kinetics of the blue shift (Vmax =At))are clearly biexponential, and good fits are obtained with the k12 and k23 values obtained as described above (see Table 1). However from these results it is difficult to ascertain 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 stepwise blue shift of the transient absorption (Figures l b and 4) monitored by the ligand exchange in the coordination sphere of the calcium cation. Furthermore the close similarity of the S2 spectrum (Figure 3) with that of the DE state (SI spectrum in Figure 2) seems to prove that the TICT state is formed during the step leading to the L*IS/M species. Decreasing of the rate constant k23 with polarity of the solvent (see Table 1) is in favor of a ligand exchange in a reaction where the less-polar solvent (BuCN), the more aggregated, is the fastest exchanged. We may observe however that the absorption spectrum of the so-called (L*/S/M) species (spectrum S3 in Figure 3) is not
J. Phys. Chem., Vol. 99,No. 43, 1995 15713
Letters
TABLE 1: Rate Constants of the Different Processes Following Excitation of DCS-crown (Scheme 1) and DCS-crown-Ca2' Complex (Scheme 2)" sample DCS-crown
solvent kiz BuCN (2.5 f 0.5) x 10" CH3CN DCS-crown/Ca2+ BuCN CH3CN
kzo (s- ) (2 f 0.1) x lo9 (4.2 f 0.2) x 109
k12(~-')
k23
(3.3 f 0.8) x 10" (2.5 f 0.5) x 10"
The klo and k20 rate constants are assumed to be negligible compared with simulated curves of Figure 4a.
exactly like the absorption spectrum of the TICT state at long times after excitation (spectrum S 2 in Figure 2). A similarity is however expected if we consider that in (L*/S/M) the interaction of the calcium ion with the nitrogen atom of the DCS-crown has been suppressed by the interposition of a solvent molecule. In our kinetic model we did not take into account a possible equilibrium between the populations of the (L*M) and of the (L*/S/M) state. Taking into account such an equilibrium leads however to a good fit too but was not envisaged, it being more reasonable to consider the simplest model which accounts for the results within the experimental error. However such an equilibrium is quite reasonable since several examples of interconversion between contact and solvent separated ion pairs have been shown:I8 for example, an isosbestic point between two absorption bands assigned to the calcium complex and the free probe in DCM-crown has been observed re~ent1y.I~In this case the absorption spectrum of the DCS-crown/Ca2+ complex should correspond at equilibrium to a linear combination of the (L*M) and (L*/S/M) absorption spectra. Assuming that the absorption spectrum of (L*/S/M), where the solvent molecules shields the probe from the cation interaction, can be represented by the absorption spectrum of the free probe, we tried to fit spectrum S 3 as a combination of the SZ state absorption spectrum (L*M state) of DCS-crown/Ca2+ complex and of the S2 state absorption spectrum of DCS-crown (TICT state). Figure 4 shows that, although not perfect, the fit gives a quite reasonable result which supports the hypothesis of the equilibrium. Although the decomplexation of the cation following the electronic excitation involves at least as many steps as there are ligands in the coordination sphere, the stability of the shape of the transient spectra after some hundreds of picoseconds and the good fit obtained with only three excited states show that at times longer than 300 ps, the influence of the calcium ion on the transient absorption spectrum is negligible and that the behavior of the probe is quite the same as that of the molecule without calcium. It is thus impossible to probe the last steps of the cation release from changes of electronic absorption.
kl2
(s-')
(4 & 0.8) x 1O1O (1.4 f 0.4) x 1 Ol0
and
k23
k30 ( $ - I )
t l (PSI
4.1 51 =(2.15 f 0.05) x lo9 3 f 1 ~ ( 1 . 8f 0.06) x lo9 4 f 1
respectively. tl and
t 2
t z (PS)
25 f 5 70 f 20
are the decay constants for the
In conclusion we have shown that with a more polar D-A stilbene (DCS-crown relative to PDS-crown) the strength of the intramolecular charge transfer (ICT) is increased and leads faster to TICT state. When a calcium cation is bound to the electrondonor substituent, the rate of the ICT process is slowed down and goes through at least two excited intermediate states distinguished by their absorption spectra. References and Notes (1) Loupy, A.; Tchoubar, B; Astruc, D. Chem. Rev. 1992, 92, 1141. (2) Mattay, J.; Vondenhof, M. Topics in Current Chem. 1991, 159, 219. (3) Piotrowiak, P.; Kobetic, R.; Schatz, T.; Strati, G. J. Phys. Chem. 1995, 99, 2250. (4) Huppert, D.; Ittah,V.; Kosower, E. M. ChemPhys. Lett. 1989,159, 267. ( 5 ) Chapmann, C. F.; Maroncelli, M. J. Phys. Chem. 1991, 95, 9095. (6) Barzoukas, M.; Blanchard-Desce, M.; Josse, D.; Lehn, J.-M.; Zyss, J. Chem. Phys. 1989, 133, 323. (7) Letard, J.-F.; Lapouyade, R.; Rettig, W. Pure Appl. Chem. 1993, 65, 1705. (8) Rettig, W.; Lapouyade, R. Topics in Fluorescence Spectroscopy, Lakowicz, J. R., Ed.; Plenum Press: New York, 1994; Vol. 4,p 109. (9) Omt, M.; Bernard, J. Phys. Rev. Lett. 1990, 65, 2716. (10) Kador, L.; Home, D. E.; Moerner, W. E. J. Phys. Chem. 1990, 94, 1237. (11) Liihr, H. G.; Vogtle, F. Acc. Chem. Res. 1985, 18, 65. (12) Fluorescent Chemosensors for ion and molecule recognition; Czarnik, A. W., ed.; American Chemical Society: Washington DC, 1992. (13) Dumon, P.; Jonusauskas, G.; Dupuy, F.; Pee, Ph.; Rullibre, C.; Ldtard, J. F,.Lapouyade, R. J. Phys. Chem. 1994, 98, 10391. (14) Lapouyade, R.; Czescha, K.; Majenz, W.; Rettig, W.; Gilabert, E.; Rullibre, C. J. Phys. Chem. 1992, 96, 9644. (15) Sida, T.; Hamill, W. H. J. Chem. Phys. 1966, 44, 2375. (16) Rettig, W.; Majenz, W.; Lapouyade, R.; Haucke, G. J. Photochem. Photobiol. A: Chem. 1992, 62, 415. (17) Martin, M. M.; Plaza, P.; Meyer, Y. H. Chem. Phys. 1995, 192, 367. (18) Peters, K.; Li, B. J. Phys. Chem. 1994, 98, 401. (19) Martin, M. M.; Plaza, P.; Dai Hung, N.; Meyer, Y. H.; Bourson, J.; Valeur, B. Chem. Phys. Lett. 1993, 202, 425. JP951328V
