Terbium Luminescence in Langmuir−Blodgett ... - ACS Publications

Langmuir 1999, 15, 613-617

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Terbium Luminescence in Langmuir-Blodgett Films of Octafunctionalized Calix[4]resorcinarenes Philip J. Dutton* and Lisa Conte Chemistry and Biochemistry School of Physical Sciences, University of Windsor, 401 Sunset, Windsor, Ontario, Canada N9B 3P4 Received June 10, 1998. In Final Form: November 18, 1998 Calix[4]resorcinarenes functionalized with pendant R-acetates or R-(diethyl acetamides) [(n-C11H23)4C[4]R-(OCH2COX)8; 1, X ) OMe; 2, X ) NEt2] have been shown to incorporate terbium(III) ions into Langmuir-Blodgett films. Energy-transfer luminescence from a single monolayer on quartz has been demonstrated. An excitation wavelength of 280 nm, corresponding to the chromophore of the ligands aromatic rings, was used. Emission was observed at 489, 545.5, 586, and 620 nm, corresponding to the known transitions of terbium (5D4 f 7F6, 5D4 f 7F5, 5D4 f 7F4, and 5D4 f 7F3, respectively). It was demonstrated that a multilayer LB film of 1 and a monolayer film of 2 readily abstract terbium(III) from 2 × 10-4 M aqueous solutions of TbCl3 and that energy-transfer luminescence was subsequently exhibited.

Introduction Luminescent lanthanide ions have unique characteristics which have led to extensive studies of their properties in recent years.1,2 Long luminescent lifetimes and the narrow emission bands have allowed the use of luminescent lanthanide cations in label technology for clinical diagnostic use.3,4 The “antenna effect” has been exploited in lanthanide complexes in which light is absorbed by the encapsulating ligand and energy is transferred to the emitting metal cation.2,5,6 Encapsulating ligands have included families of compounds such as cryptands, coronands, podands, functionalized calixarenes, and cyclodextrins.1,2,5 Using infrared, Raman, and UV spectrophotometric techniques, we have recently demonstrated that octasubstituted calix[4]resorcinarenes form LangmuirBlodgett (LB) films in which univalent and bivalent metal cations are directly associated with the pendant binding sites of the molecules in the LB film.7 This phenomenon led to this investigation of a new system for energy-transfer luminescence in Langmuir-Blodgett films. This work is the first report of the incorporation of trivalent metal cations into Langmuir-Blodgett films of any of the calix family of compounds. It is demonstrated that Langmuir-Blodgett films of resorcinarenes can abstract terbium(III) ions from aqueous solution. The calix[4]resorcinarenes that were used in this study are shown in Figure 1 in the flattened cone conformation. This conformation has been demonstrated to be important in both solutions and LB films.7 A few examples of lanthanide luminescence in Langmuir-Blodgett films have been reported in the literature, * Corresponding author. Phone: 519-253-4232 ext 3549. Fax: 519-973-7098. E-mail: [email protected]. (1) da Silva, A. P.; Gunaratne, H. Q. N.; Gunnlaugsson, T.; Huxley, A. J. M.; McCoy, C. P.; Tademacher, J. T.; Rice, T. E. Chem. Rev. 1997, 97, 1515-1566. (2) Sabbatini, N.; Guardigli, M.; Lehn, J.-M. Coord. Chem. Rev. 1993, 123, 201-228. (3) Sabbatini, N.; Mecati, A.; Guardigli, M.; Balzani, V.; Lehn, J.-M.; Zeissel, R.; Ungaro, R. J. Luminesc. 1991, 48 & 49, 463-468. (4) Hemmila¨, I. J. Alloys Compounds 1995, 225, 480-485. (5) Rudainski, C. M.; Hartmann, W. K.; Nocera, D. G. Coord. Chem. Rev. 1998, 171, 115-123. (6) Serra, O. A.; Rosa, I. L. V.; Medeiros, C. L.; Zaniquelli, M. E. D. J. Luminesc. 1994, 112-114. (7) Moreira, W. C.; Dutton, P. J.; Aroca, R. Langmuir 1995, 11, 31373144.

Figure 1. Octafunctionalized calix[4]resorcinarenes.

all of which use β-diketonate ligands.6,8-10 The majority report energy-transfer luminescence from 50 LangmuirBlodgett layers deposited either from mixed Langmuir films of β-diketonate complexes with octadecane or fatty acids9-11 or from Langmuir films of pure β-diketonate complexes.10 Serra has identified energy-transfer luminescence from a single monolayer of dihexadecyl phosphate europium(III) activated by dipping the LB film into a benzoyltrifuoroacetone solution at pH 6.5.6 Other systems have been identified in which lanthanides have been incorporated into LB films for purposes other than energy-transfer luminescence. For example, diphthalocyanine films have been used to study electrochromic and electrochemical behavior12 and a hemicyanine complex of europium has been observed to exhibit good second-harmonic generation.13 Lanthanide ions have been shown to affect monolayer and multilayer properties when incorporated into LB films of some unsaturated fatty acids.14 (8) Serra, O. A.; Nassar, E. J.; Calefi, P. S.; Rosa, I. L. B. J. Alloys Compounds, in press. (9) Qian, D.-J.; Nakahara, H.; Fukada, K.; Yang, K.-Z. Langmuir 1995, 11 (1), 4491-4494. (10) Wang, K.-Z.; Huang, C.-H.; Gao, L.-H.; Song, J.-Q.; Xu, G.-X.; Xu, Y.; Liu, Y.-Q.; Zhu, D.-B. J. Rare Earths-Special Issue 1995, 522525. (11) Qian, D.; Nakahara, H.; Fukuda, K.; Yang, K. Chem. Lett. 1995, 175-176. (12) Liu, Y.; Shigehara, K.; Hara, M.; Yamada, A. J. Am. Chem. Soc. 1991, 113, 440-443. (13) Zhou, D. J.; Huang, C. H.; Wang, K. Z.; Xu, G. X. Langmuir 1994, 10, 1910-1912. (14) Linde´n, M.; Rosenholm, J. B. Langmuir 1995, 11, 4499-4504.

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There have been several reports of energy-transfer luminescence from calixarene-lanthanide systems in both solution3,15-19 and the solid state.20 Shinkai21 has reported a novel method for the discrimination of the energytransfer route exhibited by calixarene systems in solution and has observed energy-transfer luminescence at the airwater interface in Langmuir films of amphiphilic calixarene derivatives. To date there have been no reports of calixarene systems exhibiting energy-transfer luminescence in Langmuir-Blodgett films, although it should be expected, on the basis of our results and Shinkai’s results on floating monolayers, that this could be performed. To exploit the useful properties of energy-transfer luminescence of calixarenes, these species must be successfully incorporated into a functional device. The LangmuirBlodgett technique is an excellent method for such incorporation. In aqueous solution conventional ligands do not exhibit strong coordination to lanthanide ions because of the ions’ electronic configuration. Although solvent molecules have the potential of competing effectively with ligands for lanthanides, we felt that there was a good possibility of incorporating terbium cation into the LB film in a manner similar to that observed for other metal cations on the basis of the association constants that were previously determined.22 We were not disappointed in our expectations. Terbium(III) cation is clearly shown to be present in the LB films of resorcinarenes 1 and 2 lifted from dilute terbium chloride solutions onto quartz slides, as indicated by the strong luminescence signals which have been observed. Experimental Section C-Undecylcalix[4]resorcinarene-octa-R-(methyl acetate) (1) and -octa-R-(N,N-diethyl acetamide) (2) were prepared as previously described.7,22 The solvent used to prepare the spreading solutions was spectrophotometric grade toluene from Aldrich. Terbium chloride hexahydrate (99.999%) and lanthanum chloride heptahydrate (99.999%) were obtained from Aldrich and were used without further purification. Monolayers were spread onto a Lauda Langmuir film balance equipped with a Lauda Fl-1 electronically controlled dipping device using 2 × 10-4 M solutions of 1 or 2 in toluene. The subphases were double-distilled water, passed through a Milli-Q Plus filtration system, with a final resistivity of 18.2 MΩ cm or solutions of 2 × 10-4 M TbCl3 or 2 × 10-4 M LaCl3 in Milli-Q filtered water. All π-A isotherms were recorded at both 15 and 25 °C after a waiting period of 30 min from initial spreading to allow solvent evaporation and interaction with the subphase. The barrier velocity was 2 mm min-1, which corresponded to compression rates of 0.012 and 0.025 nm2 molecule-1 min-1 for 1 and 2, respectively. A single monolayer of 2 (01 LB; note, 01 LB on each side of the substrate) was transferred from a pure aqueous subphase to optical quartz, as previously reported. Multilayer films of 1 (10 LB; 10 LB on each side of the substrate) on quartz were also prepared from pure aqueous subphases. Single-monolayer LB (15) Sabbatini, N.; Guardigli, M.; Mecati, A.; Balzani, V.; Ungaro, R.; Ghidini, E.; Casnati, A.; Pochini, A. Chem. Commun. 1990, 878879. (16) Sato, N.; Yoshida, I.; Shinkai, S. Chem. Lett. 1993, 1261-1264. (17) Sato, N.; Shinkai, S. J. Chem. Soc., Perkin Trans. 2 1993, 621624. (18) Casnati, A.; Fischer, C.; Fuardigli, M.; Isernia, A.; Manet, I.; Sabbatini, N.; Ungaro, R. J. Chem. Soc., Perkin Trans. 2 1996, 395399. (19) Beer, P. D.; Drew, M. B. B.; Grieve, A.; Kan, M.; Leeson, P. B.; Nicholson, G.; Ogden, M. I.; Williams, G. Chem. Commun. 1996, 11171118. (20) Hazenkamp, M. F.; Blasse, G.; Sabbatini, N.; Ungaro, R. Inorg. Chim. Acta 1990, 172, 93-95. (21) Matsumoto, H.; Ori, A.; Inokushi, F.; Shinkai, S. Chem. Lett. 1996, 301-302. (22) Fransen, J. R.; Dutton, P. J. Can. J. Chem. 1995, 73, 22172223.

Dutton and Conte

Figure 2. π-A isotherms of 1 using toluene as spreading solvent on 2 × 10-4 M aqueous TbCl3 or LaCl3 subphases at 15 and 25 °C. films of 1‚Tb3+ or 2‚Tb3+ on quartz substrates were prepared from 2 × 10-4 M TbCl3 subphases. In all cases the LB monolayers were transferred in the vertical mode at a deposition rate of 2 mm min-1 and a surface pressure of 25 mN m-1. A sample of 01 LB of 2 was dipped into a solution of 2 × 10-4 M TbCl3 for 10 s. The luminescence (λex ) 280 nm, λem ) 545 nm) was monitored before and after dipping. A sample of 10 LB of 1 on quartz was dipped into a 2 × 10-4 M solution of TbCl3 for various periods of time. The luminescence (λex ) 280 nm, λem ) 545 nm) was recorded between each dipping of this sample. Electronic spectra were recorded on a Response UV-vis spectrophotometer or on an SLM Aminco DW 2000 UV-vis spectrophotometer. The spectra were recorded from 01LB of 1‚ Tb3+ and 2‚Tb3+ and 10 LB of 1 before and after dipping in 2 × 10-4 M TbCl3 solution. Spectra were also collected for solutions (approximately 10-5 M) of the free ligands and complexes in 10% MeOH in CH3CN or in 10% CD3OD in CH3CN. Fluorescence spectra were recorded on either an SPEX Fluorolog spectrometer (angle of incident radiation 55° from normal, A. G. Szabo), equipped with a circulating water bath maintained at 25 °C, or an SPEX Fluorolog II spectrometer (angle of incident radiation 22° from normal, O. Serra), which was housed in an air-conditioned room maintained at ∼24 °C. Fluorescence lifetime measurements were made by O. Serra. In all cases a KV 3-70 filter was placed in the emission path to improve the spectrum quality. Reflected incident laser light was not impinging on the emission slit in either instrument. All emission spectra were recorded using an excitation wavelength of 280 nm. All excitation spectra were recorded using an emission wavelength of 545 nm, corresponding to the 5D4 f 7F5 transition for terbium(III).

Results and Discussion The isotherms for 1 and 2 on pure water and terbium chloride subphases are shown in Figures 2 and 3, respectively. The limiting area per molecule for 1 was found to be 1.95 nm2 molecule-1 on the terbium(III) subphase in contrast to the results obtained using other cations, in which the limiting area was relatively constant at ∼1.63 nm2 molecule-1.7 The shapes of the isotherms for 1 in the presence of terbium(III) were similar at both temperatures but were significantly different from those observed with other metal cations, in which it was common to observe examples of liquid-expanded to liquid-condensed phase transitions.

LB Films of Octafunctionalized Calix[4]resorcinarenes

Figure 3. π-A isotherms of 2 using toluene as spreading solvent on 2 × 10-4 M aqueous TbCl3 or LaCl3 subphases at 15 and 25 °C.

Figure 4. Stability of a floating monolayer of 2 on a 2 × 10-4 M TbCl3 subphase. The molecular area axis represents the compression phase of the experiment. After reaching 25 mN/ m, the barrier movement was halted and the surface pressure was monitored with time (top axis).

The isotherm observed for 2 on a terbium(III) subphase was also different from those of previously reported work, in which the limiting areas with uni- or divalent metal cations in the subphase ranged from 2.6 to 3.1 nm2 molecule-1.7 Although there was no significant change in the shape of the isotherm for 2 on a 2 × 10-4 M Tb3+ subphase, the limiting area was found to be independent of temperature and had a value which was significantly smaller (2.3 nm2) than that previously observed. As with previous work, no liquid-expanded to liquid-condensed phase transition was observed for 2. The stability of the floating Langmuir monolayers of 1 on the 2 × 10-4 M TbCl3 subphase was monitored after compression to 25 mN m-1, as shown in Figure 4. There was very little fluctuation in surface pressure over an 8 h period (while the barrier was kept stationary), which is indicative of a very stable Langmuir film. Langmuir films of 1 and 2 were in liquid-condensed phases at 25 mN m-1 surface pressure whether or not

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terbium(III) was present in the subphase. Efficient transfer (transfer ratio τ ) 1) of the floating Langmuir films to form Langmuir-Blodgett films on quartz was performed at 25 mN m-1. On the pure aqueous subphase, the resorcinarenes exhibited behavior that was identical with that reported previously.7 In the preparation of Langmuir-Blodgett films from terbium(III) subphases, the deposition of the first monolayer of 1 or 2 onto quartz proceeded, for the most part, as had been observed in previous cases when metal cations were present in the subphase.7 Transfer ratios in both cases were unity. There were two experimental differences observed. First there was little or no equilibration of the Langmuir film after the initial barrier compression. Typically, after spreading a resorcinarene monolayer and waiting 20 min for solvent evaporation and interaction of the resorcinarenes with the subphase, barrier movement would be initiated and the barrier would move to approximately where the monolayer was well packed, at a surface pressure of 25 mN m-1. It was then common to observe some equilibration, over a period of 10-15 min, during which the monolayer would compress slightly. In the cases of Langmuir films of 1 or 2 on terbium(III) subphases, the barrier moved to a position and came to a complete stop. No further barrier movement was observed over a 10-15 min period until transfer of the LB film was initiated by lifting the substrate. This suggests a fairly strong interaction of 1 and 2 with the subphase and the lack of the ability of the molecules in the monolayer to adjust their initial packing orientation. It was only possible to obtain a single monolayer of 1‚ Tb3+ on quartz, where in previous cases it had been possible to obtain multilayer films of 1‚Mn+ (n ) 1 or 2). The change in transfer behavior is due to the difference in the interaction of 1 with the subphase. As discussed above, a single LB monolayer was transferred from the Tb3+ solution to the quartz on the first upstroke. This type of transfer orients the headgroup (and the associated metal cation) toward the quartz surface and the hydrophobic tail section away from the surface. Transfer of the second LB was expected on the subsequent downstroke of the dipping device, which would have oriented the second layer of 1‚Mn+ in a tail-to-tail fashion with the first layer. The fact that the second layer could not be deposited indicates that the hydrophobic-hydrophobic interaction of the two layers could not compete effectively with the interaction of the hydrophilic headgroup with the subphase. The alternative head-to-tail orientation was not observed. The exact nature of the interaction with the subphase that is causing the failure of the transfer of subsequent layers is unknown at this time. Two possibilities for the change in behavior must be considered: interaction of solvent with the resorcinarene-bound lanthanide ion or electrostatic interaction of the charged complex with solvated counterions in the subphase. Experiments involving altering the counterion are required in order to explore this effect further. Structural modification of the resorcinarenes is also underway in order to address the potential of modifying the hydrophobic-hydrophobic interactions between the monolayers. Although only a single LB monolayer could be lifted, excellent luminescence signal strength was obtained from single LB layers of both 1‚Tb3+ and 2‚Tb3+. The energy-transfer luminescence of 1, 1‚Tb3+, 2, and 2‚Tb3+ was studied in 10% methanol/acetonitrile solutions. The solvent mixture was required, since the amide complex did not dissolve in pure acetonitrile. The solid complexes were prepared as previously described,15 and 6.7 × 10-5 and 8.8 × 10-5 M solutions of 1‚Tb3+ and 2‚Tb3+,

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Figure 5. Absorption spectra of (a) 01 LB of 1‚Tb3+, (b) 1.0 × 10-5 M 1‚Tb3+ in 10% MeOH/CH3CN, (c) 01 LB of 2‚Tb3+, and (d) 1.0 × 10-5 M 2‚Tb3+ in 10% MeOH/CH3CN.

respectively, in 10% methanol/acetonitrile were prepared for fluorescence measurements in order that the optical density at 280 nm was approximately 0.1. Solutions of 1 and 2 with and without 1 equiv of Tb3+ at concentrations of 5.0 × 10-5 M were also prepared. There was no significant difference in the solution absorption spectra of the two ligands with or without Tb3+ present. Figure 5 shows a comparison between the absorption spectra of the solutions and the LB films of the ligands in the presence of Tb3+. There were no notable differences in the spectra as a result of changing the complexing functionality from ester to amide, and the spectra are very similar to those obtained for the free ligands. The similarity between the solution spectra and the monolayer film spectra indicates that similar species are present in both solutions and LB films. It is well-known that the presence of suitable vibrational modes in solvents allows a nonradiative relaxation pathway for the excited state of lanthanide complexes.3,23 The OH oscillator is such a species, and when it is present in the first solvation shell of the lanthanide ion, the lanthanide emission is reduced compared to the emission observed in deuterated solvent where the lower energy OD vibration provides a less efficient relaxation pathway. Upon changing the solvent to deuterated solvent, an increase in signal intensity of 30-40% was observed at a 1:1 ratio of either ligand to Tb3+. These data indicate that solvent is participating in the solvation sphere of both 1:1 complexes in solution. The same four bands at 489, 545.5, 586, and 620 nm that were observed in the LB films of 1‚Tb3+ and 2‚Tb3+ (Figure 6) were observed in solution and correspond to the known16 terbium(III) emission lines (5D4 f 7F6, 5D4 f 7 F5, 5D4 f 7F4, and 5D4 f 7F3 transitions, respectively). The signal-to-noise ratio in the LB films is excellent, considering that the fluorescence signal is coming from only 01 LB. Energy-transfer luminescence was positively demonstrated in both solution and LB films by comparing the absorption spectra of the complexes with the excitation scans. The fact that the excitation spectrum of the complex (23) Horrocks, W. D.; Sudnick, D. R. Acc. Chem. Res. 1981, 14, 384392.

Dutton and Conte

Figure 6. Emission spectra of LB films (λex ) 280 nm): (a) 01 LB of 1‚Tb3+; (b) 01 LB of 2‚Tb3+.

Figure 7. Excitation (λem ) 543 nm) and absorption spectra of LB films of 01 LB of 1‚Tb3+ (a and b, respectively) and 01 LB of 2‚Tb3+ (c and d, respectively).

corresponds to the free ligand absorption indicates that the pathway that results in the emission begins with excitation of a ligand π-π* band. Subsequent energy transfer to the terbium ion occurs, followed by emission at wavelengths characteristic of the cation. We also noted that there is a decrease in ligand fluorescence in the terbium complex compared to the free ligand, again indicating that energy is transferred from the ligand to the metal cation. Figure 7 shows the excitation spectra of LB films and the absorption spectra of 0.5 × 10-5 M solutions of 1‚Tb3+ (a and b, respectively) and 2‚Tb3+ (c and d, respectively). Curve e of Figure 7 is the excitation scan of 01 LB of 1 lifted onto quartz from a pure water subphase. The drift in curve e is due to a small amount of emission from the filter and is minimized by keeping slit widths low. It is noted that an apparent 5 nm blue shift occurs in the excitation spectra compared to the absorption spectra of both complexes. It is clear that there is a significantly higher baseline on the short wavelength end of the

LB Films of Octafunctionalized Calix[4]resorcinarenes

spectrum. This is a characteristic of scattered light which is a common problem when monitoring weak fluorescent signals. The scatter of radiation falls approximately as the inverse of the fourth power of the wavelength. When one adds such an exponential to a Gaussian curve, one sees a shift in the apparent maximum of the curve to shorter wavelength by 4 or 5 nm. The emission lifetimes of the LB films were determined. The lifetimes for 01 LB of 1‚Tb3+ and 10 LB of 1‚Tb3+ were very similar to each other with values of 0.91 and 0.88 ms, respectively. The lifetime of 2‚Tb3+ was 1.25 ms, about 30% longer than that of the ester. These lifetimes are comparable with the lifetimes observed for the related calixarenes.15,20 The 2‚Tb3+ lifetime is slightly lower than that reported for the calixarene amide‚Tb3+ complex in aqueous solution (1.5 ms)15 and is comparable to the lifetime of the calixarene amide‚Eu3+ luminescence observed in the solid state.20 The lifetimes do not indicate complete encapsulation of the terbium ion, and thus there is still some solvent interaction and resultant quenching of the energy-transfer luminescence. These data are in agreeement with the solution data which indicate solvent participation in the Tb3+ solvation sphere of the 1:1 complex. Since LB films of 1 or 2 on quartz had the potential of abstracting terbium(III) from aqueous solution, samples were dipped in 2 × 10-4 M TbCl3 solution after obtaining initial luminescence (i.e. blank) and UV-vis spectra. Luminescence spectra taken after the Tb3+ dip confirmed that terbium(III) was bound to the film and that energytransfer luminescence was observed. Following this result, we began to examine the terbium(III) uptake that was exhibited by Langmuir-Blodgett films of 1. Since it was clearly shown that terbium luminescence was exhibited after dipping the film into a dilute Tb3+ solution, it was of interest to examine what the time course of the uptake was in a multilayer film. A quartz slide with 10 LB of 1 on each side was dipped into an aqueous 2 × 10-4 M TbCl3 solution for various periods of time. Between each dip, the fluorescence spectrum was acquired, and the sample was dipped again. Figure 8 shows a plot of fluorescence intensity at 545 nm (λex ) 280 nm) versus total time that the sample was dipped in the solution. There is clearly an uptake of terbium(III) over the course of 1 h. About 75% of the maximum fluorescence intensity was obtained after only 5 min in solution. It is unknown at this time if there is diffusion of the terbium ions deep into the film or if this

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Figure 8. Fluorescence intensity versus time for 10 LB of 1 dipped in aqueous TbCl3 (2 × 10-4 M) (λex ) 280 nm, λem ) 545 nm).

is simply a surface phenomenon. The terbium uptake of the LB films will be examined fully and reported in a future publication. Conclusions It has been clearly shown that octasubstituted resorcinarenes act as ligands for terbium(III) and that energytransfer luminescence is exhibited by these systems. Terbium ion is bound by the resorcinarenes into Langmuir-Blodgett films, and strong luminescence signals are observed for single monolayers on quartz. Acknowledgment. The authors would like to thank A. G. Szabo, University of Windsor, and O. A. Serra, Universidade Sa˜o Paulo at Ribera˜o Preto, Brazil, for the use of their fluorescence spectrometers and the technical help which both provided. Funding for this project was provided by NSERC and the University of Windsor. P.J.D. would like to thank the University of Windsor Academic Development Fund for a Travel Grant which made the visit to Brazil possible. LA980678G