Photoluminescence of a Covalent Assembled Porphyrin-Based

Aug 2, 2006 - Definition and Use of the Experimental Sensible Parameters To Characterize Sensitivity and Precision of a Generic Oxygen Optical Sensor ...
0 downloads 13 Views 168KB Size
Download PDF
J. Phys. Chem. B 2006, 110, 16781-16786

16781

Photoluminescence of a Covalent Assembled Porphyrin-Based Monolayer: Optical Behavior in the Presence of O2 Antonino Gulino,*,† Salvatore Giuffrida,† Placido Mineo,† Michele Purrazzo,† Emilio Scamporrino,† Giorgio Ventimiglia,† Milko E. van der Boom,*,‡ and Ignazio Fragala` *,† Dipartimento di Scienze Chimiche, UniVersita` di Catania, and I.N.S.T.M. UdR of Catania, Viale Andrea Doria 6, 95125 Catania, Italy, and Department of Organic Chemistry, Weizmann Institute of Science, RehoVot 76100, Israel ReceiVed: May 15, 2006; In Final Form: June 22, 2006

The optical O2 recognition capability of a covalently assembled monolayer (CAM) of 5,10,15-tri-{p-dodecanoxyphenyl}-20-(p-hydroxyphenyl) porphyrin on silica-based substrates was studied at room temperature by both UV-vis and photoluminescence (PL) measurements. The optical properties of this robust monolayer setup appear to be highly sensitive to the O2 concentration in N2. Both UV-vis and PL measurements were used to study the porphyrin-oxygen interactions. The monolayer-based sensor exhibits a short response time and can be restored within seconds. The oxygen-induced luminescence quenching of the monolayer involves both ground and excited states. The proposed mechanism responsible for the luminescence quenching involves different kinds of interactions between the monolayer and O2.

Introduction

Experimental Section

The prospect of reversible detection of chemicals at low concentrations has become fundamental for the fabrication of highly sensitive gas sensors.1-7 Determination of molecular oxygen concentrations has important applications in many different areas ranging from environmental monitoring to biological, medical, analytical, and industrial chemistry. Many O2 sensors have been reported, including electrical systems based on semiconducting metal oxide films.8-10 Other O2 sensors have been realized by using photosensitive systems.11,12 With respect to amperometric methods, luminescence-based detection methods show higher sensitivity, do not consume O2, and can be combined with optical fibers for remote sensing. Frequently used dyes include macrocycles,13-17 polycyclic aromatic hydrocarbons,18-20 and transition metal complexes.21,22 In addition, monolayer assemblies of “hinged” iron-porphyrins on semiconductor surfaces as potential O2 sensors are known.23 There is an increasing technological interest in the synthesis of hybrid inorganic/organic nanomaterials by covalent bonding of organic molecules on suitable inorganic surfaces for the fabrication of devices showing specific molecular properties.1,24-39 In this context, 5,10,15-tri-(p-dodecanoxyphenyl)20-(p-hydroxyphenyl) porphyrin40,41 (P) shows a good affinity toward O2 (vide infra), making it an ideal candidate for the fabrication of a monolayer-based optical gas sensor. Its design prevents chromophore-chromophore aggregration,42-44 allowing analytes to interact with the porphyrin core. Therefore, there is enough motivation for an extensive investigation on the optical recognition behavior of O2 by a monolayer of this porphyrinbased molecular building block covalently assembled on silicabased substrates (Chart 1). The optical gas sensor presented here is based on the reversible luminescence quenching of the porphyrin chromophore by molecular oxygen.

The previously reported functional monolayers are formed by solution-based covalent assembly of 5,10,15-tri-(p-dodecanoxyphenyl)-20-(p-hydroxyphenyl) porphyrin at an organic interface.37 Siloxane-based coupling layers were prepared on freshly cleaned substrates using commercially available pchloromethyl-phenyltrichlorosilane (Aldrich). Subsequently, the known porphyrin building block was reacted with the benzylhalide-terminated coupling layer. Similar surface coupling reactions of benzyl halide interfaces with phenols or pyridine moieties to form ether linkages or pyridinium salts, respectively, have been reported.45 These siloxane-based monolayers strongly adhere to the glass and silicon substrates, are insoluble in common organic solvents, and cannot be removed by the “Scotch-tape decohesion” test. The freshly prepared P-CAM monolayer setup has been characterized by angle-resolved X-ray photoelectron spectroscopy (ARXPS), optical (UV-vis) measurements, and both static and dynamic contact angle (CA) measurements.37 Room-temperature photoluminescence (PL) spectra of both porphyrin solution and P-CAM were obtained with a SPEX Fluorolog 111 instrument equipped with a xenon lamp (450 W) operating in the 200-800 nm range. The P-CAM was fixed into a fluorescence cuvette.46 Samples were photoexcited with a 428 nm line beam. The emission was recorded at 90° with respect to the exciting line beam. Controlled atmospheric measurements were performed allowing the appropriate gas mixture to flow in the cuvette sealed with a closely fitting suba-seal rubber lid equipped with two (IN and OUT) needles.46 Flow rates were controlled within (1 sccm using MKS flow controllers and a MKS 147 multigas controller.

* Author to whom correspondence should be addressed. E-mail: [email protected]; [email protected]. † Universita ` di Catania and I.N.S.T.M. UdR of Catania. ‡ Weizmann Institute of Science.

Results and Discussion Figure 1 shows a representative transmission UV-vis spectrum of the P-CAM sensor on a glass substrate. The characteristic Soret band at λmax ) 428 nm is evident and is nearly identical to that observed in the spectrum of the porphyrin in a

10.1021/jp062967g CCC: $33.50 © 2006 American Chemical Society Published on Web 08/02/2006

16782 J. Phys. Chem. B, Vol. 110, No. 33, 2006

Gulino et al.

CHART 1: 5,10,15-Tri-(p-dodecanoxyphenyl)-20-(p-hydroxyphenyl) Porphyrin Covalently Assembled on Both Sides of a Silica-Based Substrate37 a

a

The bulky groups prevent chromophore-chromophore aggregration, allowing O2 to interact with the porphyrin core in a reversible manner.

Figure 1. Representative UV-vis spectrum showing the Soret band at λmax ) 428 nm of the 5,10,15-tri-(p-dodecanoxyphenyl)-20-(phydroxy-phenyl) porphyrin-based monolayer (P-CAM) covalently assembled on glass.

solution of cyclohexane (λmax ) 421 nm, vide infra). Satellite Q-bands are of the same intensity as the noise, which is not uncommon for monolayers.37,45 The calculated average chromophore density is 6.1 × 1012 molecules/cm2. 36 To study the O2 optical detection capability of the present porphyrin system in solution, both absorption and emission measurements under different experimental conditions were performed.47 The UV-vis spectra of the porphyrin in cyclohexane (3.5 × 10-6 M) saturated with N2, air, or O2, respectively, show an evident intensity decrease of the Soret band at

Figure 2. Representative UV-vis solution spectra of the 5,10,15-tri(p-dodecanoxyphenyl)-20-(p-hydroxyphenyl) porphyrin (P) in cyclohexane (3.5 × 10-6 M). Decreasing intensity spectra refer to the N2 (a, black line), air (b, red line), and O2 (c, green line) saturated solutions, respectively. The inset shows the expanded scale.

421 nm and of the relatively small Q-bands in the 500-800 nm range (Figure 2). In particular, the reduced absorbance intensities are 94% (air) and 66% (O2) with respect to that of the same solution saturated with N2. This result agrees well with previously reported data on other systems and indicates the reversible formation of a P‚O2 adduct

PL of a Porphyrin-Based Monolayer in O2

Figure 3. Representative photoluminescence spectra of the 3.5 × 10-6 M cyclohexane solution of 5,10,15-tri-(p-dodecanoxyphenyl)-20-(phydroxyphenyl) porphyrin. Decreasing intensity spectra refer to the N2 (a, black line), air (b, red line), and O2 (c, green line) saturated solutions, respectively.

that shows a molar absorbance coefficient lower than that measured for the starting solution under N2.48-51

J. Phys. Chem. B, Vol. 110, No. 33, 2006 16783

Figure 4. Representative UV-vis spectra of the P-CAM on glass. Decreasing intensity spectra refer to the N2 (a, black line) and O2 (b, red line) saturated samples, respectively. The signal is restored upon exposure to N2. An experimental uncertainty of (3% in the intensity evaluation was taken into account because of the signal-to-noise ratio. The inset shows the expanded scale of the spectra.

P + O2 a P·O2 Similar conclusions can be drawn from PL emission measurements (λexc ) 428 nm) of the same porphyrin solutions.52-56 In fact, two strong PL emissions (at 657 and 721 nm) are always present (Figure 3). Reduced emission intensities, 84% (air) and 57% (O2), become evident with respect to the reference solution saturated with N2.13-17,52-56 These observations are in line with studies for systems forming ground-state complexes with O247-51 and confirm that O2 quenches the fluorescence by forming an adduct with the porphyrin in its ground state, even though further contributions from the excited-state quenching cannot be ruled out.57 Importantly, the system can be reversibly brought to the reference intensity value by bubbling N2 into the solution. In general, an excited state can be dynamically and/or statically quenched. In the latter case the formation of an encounter complex (exciplex), (P‚O2)*, is involved,47,58-63 whose formation strongly depends on the lifetime of the porphyrin singlet excited state.25,47,58 Interaction between O2 and the S1 excited state of many organic compounds with long singlet state lifetime has been reported.18-20 Metalloporphyrin phosphorescence quenching by O2 has also been studied.64-67 Nevertheless, for some mesoporphyrin dimethyl esters it has been reported that the relatively short lifetime of the porphyrin singlet state (19 ns) is long enough to ensure 60% quenching of the S1 state.47 Both the (P‚O2) ground state and the (P‚O2)* lowest excited S1 state complexes are likely to contribute to the observed fluorescence quenching in solution. Experiments in solution can be considered blank tests for the further rationalization of the P-CAM optical behavior. Figure 4 shows absorption spectra of the P-CAM under a 100% either N2 or O2 atmosphere. A small absorbance intensity decrease (∼13%) occurs on switching to O2. Similar to the observations in solution, a P‚O2 adduct is probably responsible for the optical changes of the monolayer.

Figure 5. Representative photoluminescence (λexc ) 428 nm) quenching of the P-CAM in N2/O2 mixtures with increasing O2 concentrations. The most intense spectrum refers to a 100% N2 atmosphere. The lowest intensity spectrum refers to a 100% O2 atmosphere. Dots refer to the spectrum restored with N2 after a cycle of measurements. The inset shows reversible spectral changes of the PL band at λmax ) 658 nm obtained by switching between N2 and O2.

Luminescence of the P-CAM also has been measured. The P-CAM exhibits PL with fluorescence emissions at 658 and 720 nm under a 428 nm excitation (Figure 5).68-72 The P-CAM excitation spectrum (Figure 6), monitored at the emission maximum (658 nm), matches well the absorption spectrum of the monolayer (Figure 1).68 Three of the four Q-bands at 515,

16784 J. Phys. Chem. B, Vol. 110, No. 33, 2006

Figure 6. Representative excitation spectrum of the P-CAM monitored at λem ) 658 nm.

555 and 600 nm, typical of the free-base porphyrin,37,68 are evident in the excitation spectrum because of the high sensitivity of the emission technique. The fourth Q-band is hidden under the harmonic band beyond the range of Figure 5. A better insight into the sensing behavior of the present P-CAM has been obtained upon comparing luminescence measurements under a 100% either N2 or O2 atmosphere. The highest emission (I0) was obtained under a N2 atmosphere, whereas a 64% emission was observed when the P-CAM was excited in a 100% O2 atmosphere (I100) (Figure 5).46 Moreover, the initial 100% intensity was fully restored under N2 (see dots in Figure 5). On the basis of these observations, there is clear evidence that the fluorescence quenching is more relevant than effects based on absorbance values, indicating that again both the ground state and the lowest excited state contribute to the quenching of the P-CAM fluorescence. Of course, in the solid state the singlet state lifetime might be longer than that in solution, thus favoring the exciplex formation. These experiments, according to earlier studies on differently substituted free-base porphyrins,48,53,57,58 clearly show the dependence of the P-CAM PL emission upon the O2 concentration. Moreover, the ratio, I0/I100 ) 1.6, represents a good benchmark for a solid-state O2 detection device based on a molecular monolayer system and provides a first indication of the recognition capability of the P-CAM.56 From this perspective, it is crucial to evaluate the upper sensitivity level (the minimum O2 percentage) of the sensor. PL measurements upon exposure to different N2/O2 mixtures showed intermediate PL intensities ranging between I0 and I100 (Figure 5), and stable PL intensity emission values are attained after only 20 s of exposure.54 The lowest O2 concentration in N2 revealed by the P-CAM corresponds to 0.2% (Figure 7). Repeat experiments with different gas mixtures are highly reproducible and reversible and indicate a good response dynamic. Figure 7 clearly shows that the maximum response occurs at very low O2 percentages (0% < O2 < 2.5%) where a linear decrease is observed.54 For higher O2 percentages a nonlinear behavior is observed. Nonlinear Stern-Volmer plots are often observed in thick films or matrix-embedded sensor molecules.54,73,74 This behavior has been associated with sensor molecules in different sites each having a specific quenching constant; therefore, several exponential decay curves are obtained.73,74 This dif-

Gulino et al.

Figure 7. Photoluminescence behavior of the P-CAM vs the O2 concentration. The inset shows the expanded scale of the linear behavior.

SCHEME 1: Possible Porphyrin-O2 Interactions

ferential quenching produces a nonlinear plot since it represents the combination of linear plots with different slopes.73 Similar behavior has been also observed in thicker films because of nonlinear O2 solubility.73 In the present system, the P-CAM consists of a single layer of a lumophore, not embedded in any polymeric support. Thus, quenching occurs in a unique twodimensional sensing surface. The nonlinear I/I0 plot (Figure 7) is consistent with the presence of at least three quenching mechanisms: (i) formation of an adduct in the ground state (P‚O2), (ii) dynamic quenching of the excited state 1P*, and (iii) static quenching by formation of the exciplex (P‚O2)*. The latter always yields a curved SternVolmer plot,63 while either the formation of an adduct in the ground state or the dynamic quenching (KSV) both involve linear Stern-Volmer plots. The possible processes are shown in Scheme 1 where K1 represents the equilibrium constant for the adduct formation in the ground state, K2 refers to the exciplex formation, and KSV is the Stern Volmer constant associated with the dynamic quenching. The overall O2 interactions (either with ground (P), excited (1P*), or both states) causes diminution of emission intensities. For low oxygen concentrations the I0/I ratio versus [O2] plot is linear as a consequence of the dynamic quenching and/or adduct formation with the ground state. Upon an increase of the O2 concentration all quenching processes become involved. The observed fluorescence intensity ratio of I0/I100 ) 1.6 implies a residual fluorescence not quenched by O2. Therefore, either some of the porphyrin-based molecular building blocks are inaccessible to the quencher or the quenching process is not efficient. It is likely that both factors play are role. The P-CAM setup is not perfectly homogeneous on a

PL of a Porphyrin-Based Monolayer in O2 molecular scale. Consequently, some chromophores may be less accessible to the quencher. Minor structural variations are likely to be expressed in the sensing and optical properties of the system. The induced steric hindrance around the porphyrin core may not entirely prevent the formation of some porphyrin aggregates on the surface, although this has not been observed by UV-vis measurements. If the dynamic quenching is the predominant mechanism, then it is not very efficient due to the relatively short lifetime of the excited state.47 This could also explain the residual fluorescence. Summary and Conclusions The 5,10,15-tri-{p-dodecanoxyphenyl}-20-(p-hydroxyphenyl) porphyrin molecular monolayer, covalently assembled to engineered silica substrates, has proven to be suited for molecular oxygen optical recognition. Photoluminescence (PL) measurements performed on the P-CAM at room temperature indicate that the system is able to reveal low O2 concentrations (0.2%) in N2. The proposed mechanism responsible for the luminescence quenching involves both ground and excited states. The O2-induced quenching behavior indicates the presence of structural variations in the monolayer structure at the molecular level. Although further studies are required, it implies that neutral inorganic gases such as O2 may be used to reveal fine details regarding the monolayer packing and/or intermolecular interactions. An advantage of the monolayer system with respect to porous multilayer films and matrixes lies in the fact that the latter can behave as sponges, which are difficult to restore. Interestingly, exposing the monolayer-based O2 sensor presented here for only a few seconds to N2 is sufficient to fully reset the system. Acknowledgment. The authors thank NATO (SfP project 981964) and the Ministero Istruzione Universita` e Ricerca (MIUR, Roma) for financial support (PRIN 2005 and FIRB 2001). M.E.v.d.B. is incumbent of the Dewey David Stone and Harry Levine Career Development Chair and head of the Minerva Junior Research Group on Molecular and Interface Design. References and Notes (1) Gulino, A.; Bazzano, S.; Mineo, P.; Scamporrino, E.; Vitalini, D.; Fragala`, I. Chem. Mater. 2005, 17, 521. (2) Chung, J.; Lee, K. H.; Lee, J.; Troya, D.; Schatz, G. C. Nanotechnology 2004, 15, 1596. (3) Wang, Z. L. AdV. Mater. 2003, 15, 432. (4) Kappler, J.; Weimar, U.; Gopel, W. In AdVanced Gas Sensing; Doll, T., Ed.; Kluwer Academic Publishers: Boston, MA, 2003. (5) Massari, A. M.; Gurney, R. W.; Wightman, M. D.; Huang, C.-H. K.; Nguyen, S. T.; Hupp, J. T. Polyhedron 2003, 22, 3065. (6) Rodriguez-Mendez, M. L.; Gorbunova, Y.; de Saya, J. A. Langmuir 2002, 18, 9560. (7) Lavigne, J. J.; Anslyn, E. V. Angew. Chem., Int. Ed. 2001, 40, 3118. (8) Eranna, G.; Joshi, B.; Runthala, D.; Gupta, R. Crit. ReV. Solid State Mater. Sci. 2004, 29, 111. (9) Ramamoorthy, R.; Dutta, P. K.; Akbar, S. A. J. Mater. Sci. 2003, 38, 4271. (10) Baraton, M. I.; Merhari, L.; Wang, J.; Gonsalves, K. E. Nanotechnology 1998, 9, 356. (11) Lu, X.; Winnik, M. A. Chem. Mater. 2001, 13, 3449. (12) Demas, J. N.; DeGraff, B. A.; Coleman, P. B. Anal. Chem. 1999, 71, 793A. (13) Zhang, H.; Sun, Y.; Ye, K.; Zhang, P.; Wang, Y. J. Mater. Chem. 2005, 15, 3181. (14) Brinas, R.; Troxler, T.; Hochstrasser, R. M.; Vinogradov, S. A. J. Am. Chem. Soc. 2005, 127, 11851. (15) Han, B.-H.; Manners, I.; Winnik, M. A. Chem. Mater. 2005, 17, 3160.

J. Phys. Chem. B, Vol. 110, No. 33, 2006 16785 (16) Koese, M. E.; Carroll, B. F.; Schanze, K. S. Langmuir 2005, 21, 9121. (17) Khalil, G.; Gouterman, M.; Ching, S.; Costin, C.; Coyle, L.; Gouin, S.; Gree, E.; Sadilek, M.; Wan, R.; Yearyean, J.; Zelelow, B. J. Porphyrins Phthalocyanines 2002, 6, 135. (18) Okamoto, M.; Tanaka, F. J. Phys. Chem. A 2002, 106, 3982. (19) Abe, T.; Matsuguchi, M.; Sakai, Y. Chem. Sens. 2000, 16, 109. (20) Olea, A. F.; Wilkinson, F. J. Phys. Chem. 1995, 99, 4518. (21) Demas, J. N.; DeGraff, B. A. Coord. Chem. ReV. 2001, 211, 317. (22) Di Marco, G.; Lanza, M.; Campagna, S. AdV. Mater. 1995, 7, 468. (23) Gonen, A.; Albena, I.; Rami, C.; Felder, C. E.; Cahen, D.; Ellis, A. B.; Shanzer, A. J. Am. Chem. Soc. 2000, 122, 1116. (24) Gulino, A.; Condorelli, G. G.; Mineo, G. P.; Fragala`, I. Nanotechnology 2005, 16, 2170. (25) Gulino, A.; Mineo, G. P.; Bazzano, S.; Vitalini, D.; Fragala`, I. Chem. Mater. 2005, 17, 4043. (26) Imahori, H.; Hosomizu, K.; Mori, Y.; Sato, T.; Ahn, T. K.; Kim, S. K.; Kim, D.; Nishimura, Y.; Yamazaki, I.; Ishii, H.; Hotta, H.; Matano, Y. J. Phys. Chem. B. 2004, 108, 5018. (27) Liu, Z.; Yasseri, A. A.; Loewe, R. S.; Lysenko, A. B.; Malinovskii, V. L.; Zhao, Q.; Surthi, S.; Li, Q.; Misra, V.; Lindsey, J. S.; Bocian, D. F. J. Org. Chem. 2004, 69, 5568. (28) Yamada, H.; Imahori, H.; Nishimura, Y.; Yamazaki, I.; Ahn, T. K.; Kim, S. K.; Kim, D.; Fukuzumi, S. J. Am. Chem. Soc. 2003, 125, 9129. (29) Fendler, J. H. Chem. Mater. 2001, 13, 3196. (30) Gomez-Romero, P. AdV. Mater. 2001, 13, 163. (31) Giribabu, L.; Anita Rao, T.; Maiya, B. G. Inorg. Chem. 1999, 38, 4971. (32) Offord, D. A.; Sachs, S. B.; Ennis, M. S.; Eberspacher, T. A.; Griffin, J. H.; Chidsey, C. E. D.; Collman, J. P. J. Am. Chem. Soc. 1998, 120, 4478. (33) Lin, W.; Lin, W.; Wong, G. K.; Marks, T. J. J. Am. Chem. Soc. 1996, 118, 8034. (34) Schick, G. A.; Schreiman, I. C.; Wagner, R. W.; Lindsey, J. S.; Bocian, D. F. J. Am. Chem. Soc. 1989, 111, 1344. (35) Worsfold, O.; Dooling, C. M.; Richardson, T. H.; Vysotsky, M. O.; Tregonning, R.; Hunter, C. A.; Malins, C. J. Mater. Chem. 2001, 11, 399. (36) Li, D. Q.; Swanson, B. I.; Robinson, J. M.; Hoffbauer, M. A. J. Am. Chem. Soc. 1993, 115, 6975. (37) Gulino, A.; Mineo, P.; Scamporrino, E.; Vitalini, D.; Fragala`, I. Chem. Mater. 2004, 16, 1838. (38) Altman, M.; Shukla, A. D.; Zubkov, T.; Evmenenko, G.; Dutta, P.; van der Boom, M. E. J. Am. Chem. Soc. 2006, 128, 7374. (39) Gupta, T.; van der Boom, M. E. J. Am. Chem. Soc. 2006, 128, 8400. (40) Vitalini, D.; Mineo, P.; Scamporrino, E. Macromolecules 1999, 32, 60. (41) Mineo, P.; Scamporrino, E.; Vitalini, D. Macromol. Rapid Commun. 2002, 23, 681. (42) The presence of only one hydroxyl group in the peripheral position of the porphyrin allows an univocal binding of this compound to benzylhalide-terminated interfaces. (43) Splan, K. E.; Hupp, J. T. Langmuir 2004, 20, 10560. (44) Micali, N.; Villari, V.; Mineo, P.; Vitalini, D.; Scamporrino, E.; Crupi, V.; Majolino, D.; Migliardo, P.; Venuti, V. J. Phys. Chem. B 2003, 107, 5095. (45) Yerushalmi, R.; Scherz, A.; van der Boom, M. E. J. Am. Chem. Soc. 2004, 126, 2700. (46) Gulino, A.; Bazzano, S.; Condorelli, G. G.; Giuffrida, S.; Mineo, P.; Satriano, C.; Scamporrino, E.; Ventimiglia, G.; Vitalini, D.; Fragala`, I. Chem. Mater. 2005, 17, 1079. (47) Bensasson, R. V.; Land, E. J.; Truscott, T. G. Excited States and Free Radicals in Biology and Medicine; Oxford Science Publications: Oxford, U. K., 1993. (48) Kuriyama, Y.; Ogilby, P. R.; Mikkelsen, K. V. J. Phys. Chem. 1994, 98, 11918. (49) Kristiansen, M.; Scurlock, R. D.; Lu, K.; Ogilby, P. R. J. Phys. Chem. 1991, 95, 5190. (50) Gooding, E. A.; Serak, K. R.; Ogilby, P. R. J. Phys. Chem. 1991, 95, 7868. (51) Ishida, H.; Takahashi, H.; Sato, H.; Tsubomura, H. J. Am. Chem. Soc. 1970, 92, 275. (52) Chirvony, V. S.; Ne`grerie, M.; Martin, J.-L.; Turpin, P.-Y. J. Phys. Chem. A 2002, 106, 5760. (53) Sagun, E. I.; Zenkevich, E. I.; Knyukshto, V. N.; Shulga, A. M.; Starukhin, D. A.; von Borczyskowski, C. Chem. Phys. 2002, 275, 211. (54) Amao, Y.; Miyashita, T.; Okura, I. J. Fluorine Chem. 2001, 107, 101. (55) Ashkenasy, G.; Ivanisevic, A.; Cohen, R.; Felder, C. E.; Cahen, D.; Ellis, A. B.; Shanzer, A. J. Am. Chem. Soc. 2000, 122, 1116. (56) Papkovsky, D. B.; Ponomarev, G. V.; Wolfbeis, O. S. Spectrochim. Acta, Part A 1996, 52, 1629.

16786 J. Phys. Chem. B, Vol. 110, No. 33, 2006 (57) Giasson, R.; Lee, E. J.; Zhao, X.; Wrighton, M. S. J. Phys. Chem. 1993, 97, 2596. (58) Balzani, V. In Supramolecular Photochemistry; Balzani, V., Ed.; NATO ASI Series: Series C, Mathematical and Physical Science 214; Reidel Publishing Company: Dordrecht, The Netherlands, 1987; p 220. (59) Okamoto, M. J. Phys. Chem. A 2000, 104, 7518. (60) Okamoto, M. J. Phys. Chem. A 2000, 104, 5029. (61) Roy, J. K.; Carroll, F. A.; Whitten, D. G. J. Am. Chem. Soc. 1974, 96, 6349. (62) Roy, J. K.; Whitten, D. G. J. Am. Chem. Soc. 1971, 93, 7093. (63) Xu, W.; Schmidt, R.; Whaley, M.; Demas, J. N.; DeGraff, B. A.; Karikari, E. K.; Farmer, B. L. Anal. Chem. 1995, 67, 3172. (64) Douglas, P.; Eaton, K. Sens. Actuators, B 2002, 82, 200. (65) Di Marco, G.; Lanza, M. Sens. Actuators, B 2000, 63, 42. (66) Mills, A.; Lepre, A. Anal. Chem. 1997, 69, 4653.

Gulino et al. (67) Cox, G. S.; Whitten, D. G.; Giannotti, C. Chem. Phys. Lett. 1979, 67, 511. (68) Yang, J. H.; Chen, Y. M.; Ren, Y. L.; Bai, Y. B.; Wu, Y.; Jang, Y. S.; Su, Z. M.; Yang, W. S.; Wang, Y. Q.; Zao, B.; Li, T. J. J. Photochem. Photobiol., A 2000, 134, 1. (69) Fudickar, W.; Zimmerman, J.; Ruhlmann, L.; Schneider, J.; Ro¨der, B.; Siggel, U.; Fuhrhop, J. H. J. Am. Chem. Soc. 1999, 121, 9539. (70) Kipp, R. A.; Li, Y.; Simon, J. A.; Schmehl, R. H. J. Photochem. Photobiol., A 1999, 121, 27. (71) Kubot, P.; Mosinger, J. J. Photochem. Photobiol., A 1996, 96, 93. (72) Spellane, P. J.; Gouterman, M.; Antipas, A.; Kimnad, S.; Liu, Y. C. Inorg. Chem. 1980, 19, 386. (73) Demas, J. N.; DeGraff, B. A.; Xu, W. Anal. Chem. 1995, 67, 1377. (74) Carraway, E. R.; Demas, J. N.; DeGraff, B. A. Anal. Chem. 1991, 63, 3, 332.