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Photophysical Study of Naphthalenophanes: Evidence of Adduct Formation with Molecular Oxygen Laura Rodrı´guez,*,†,‡ Joa˜o C. Lima,*,‡ Fernando Pina,‡ Roberta Cacciapaglia,§ Stefano Di Stefano,§ and Albert Ruggi§ Departament de Quı´mica Inorga`nica, UniVersitat de Barcelona, Martı´ i Franque`s 1-11, 08028 Barcelona, Spain, REQUIMTE, Departamento de Quı´mica, Centro de Quı´mica Fina e Biotecnologia, Faculdade de Cieˆncias e Tecnologia, UniVersidade NoVa de Lisboa, Quinta da Torre, 2825 Monte de Caparica, Portugal, and CNR and Dipartimento di Chimica, UniVersita` La Sapienza, 00185 Roma, Italy ReceiVed: July 23, 2010; ReVised Manuscript ReceiVed: October 8, 2010
The photophysical properties of two atropisomeric naphthalenophanes (1 and 2) have been studied. Their structure only differs in the relative arrangement, syn (1) or anti (2), of the two aromatic units. The compounds emission is mainly excimeric and is strongly quenched in the presence of oxygen. Comparison of emission intensities obtained from steady state and from decay times provides clear evidence of the formation of ground state charge transfer complexes between oxygen and the naphthalenophanes 1 and 2. The calculated values for the association constants are on the order of 103 M-1 (ethanol, room temperature) for both naphthalenophanes. Introduction Oxygen molecules play a fundamental role in supporting a vast range of chemical and biochemical reactions as either a reactant or a product and is essential for our daily lives. It is a very reactive and in some cases highly cytotoxic molecule that induces photodegradation processes and has significant applications in organic synthesis and in photodynamic therapy.1 For this reason, there is great effort in the design of complexes that could act as oxygen sensors or even oxygen carriers. Although the Clark electrode and its modifications are the most known and traditionally used oxygen-sensitive detectors,2,3 several systems for oxygen detection have been reported based on redox titration,4 polarography,2 reflectometry,5 UV-vis absorption spectroscopy,6 or luminescence quenching.7-9 Oxygen sensors based on the absorption or emission of light have the advantage of being noninvasive and of allowing the evaluation of oxygen content in nanometric spaces, inaccessible to macroscopic sensors. They became extremely popular in the last years10-14 and are widely used in a variety of fields including real-time clinical monitoring, environmental monitoring, food packaging technology, surface air pressure distributions, process control, organic synthesis, and photodynamic therapy.15-20 Compounds exhibiting high luminescence quantum yields and long luminescence decay times, such as the ruthenium(II) derivatives tris(4,7-diphenyl-1,10-phenanthroline)ruthenium(II) and tris(1,10-phenanthroline)ruthenium(II) are known to be good oxygen sensors, allowing the use of less sensitive detectors in the development of optical oxygen sensors.21,22 Long excited state decay times facilitate the diffusional encounter between the luminophore and the oxygen molecule and allow efficient quenching of the luminescence by a dynamic mechanism which depends on the concentration of oxygen dissolved in the solvent (or matrix) and its rate of diffusion * To whom correspondence should be addressed,
[email protected] and
[email protected]. † Universitat de Barcelona. ‡ Universidade Nova de Lisboa. § Universita` La Sapienza, Roma.
CHART 1: Atropisomeric Naphthalenophanes 1 (left) and 2 (right)
with respect to the excited state decay to the ground state. Concerning this point, it is well established that the long-lived luminescence exhibited by polycyclic aromatic hydrocarbons (naphthalene, anthracene, pyrene, etc.) is extremely sensitive to dynamic quenching by oxygen. Kenny et al.23 have recently shown evidence for the existence of contact charge transfer (CCT) complexes between oxygen and naphthalene and pyrene, based on the differences found in the Stern-Volmer slopes obtained from steady state data (I0/I vs [O2]) and time-resolved data (τ0/τ vs [O2]). The extent of dynamic quenching in aromatic molecules with long-lived luminescence, such as naphthalene or pyrene, is nevertheless very high with respect to the static quenching component (CCT complex) and only small deviations were consequently observed in Kenny’s investigations.23 In our work, the photophysical properties of the atropisomeric naphthalenophanes 1 and 2 are studied (Chart 1). These macrocycles feature pairs of aromatic moieties connected in a syn (1) or anti (2) arrangement and exhibit short-lived excimer type luminescence combined with high quantum yields. In the present case, due to the short excited state lifetimes of compounds 1 and 2, the dynamic quenching by oxygen is much less important and the quenching mechanism due to the possible formation of complexes between the naphthalenophanes and molecular oxygen can dominate.
10.1021/jp106887c 2011 American Chemical Society Published on Web 12/16/2010
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Figure 1. Normalized absorption (solid line) and emission (dotted line) spectra of 5 × 10-5 M ethanol solutions of 1 (A) and 2 (B).
Experimental Methods Materials. Compounds 1 and 2 (syn and anti isomers, respectively, of 13,17,73,77-tetramethyl-3,5,9,11-tetraoxa-1,7(1,5)dinaphthalenacyclododecaphane, (C30H32O4)) were available from previous investigations.24 Spectrophotometric grade ethanol, chloroform, and acetonitrile were used in all manipulations. For the Stern-Volmer experiments, 3 mL of 5 × 10-7 M ethanol solutions of 1 and 2 were saturated with different argon/ oxygen mixtures of different composition, previously prepared with exactly known concentrations of oxygen in argon (0, 21, 54, and 100%). In order to know the exact quantities of each gas, an empty cylinder was filled approximately with the desired quantity of oxygen, which could be exactly quantified by the calculation of the different weight between the empty cylinder and that corresponding to the first filled stage. Then the cylinder was completely filled with argon until 100% of capacity. The samples were then saturated with the preprepared mixtures. An intermediate flask with ethanol was connected between the cylinder and the sample in order to prevent solvent evaporation. The concentrations of oxygen in the saturated solutions were calculated taking into account that the solubility of this gas in ethanol at 1 atm (pure oxygen) and 25 °C is 9.92 × 10-3 M.25 Spectrophotometric and Spectrofluorimetric Measurements. Absorption spectra were recorded on a Shimadzu UV2501PC spectrophotometer and fluorescence emission spectra on a Horiba-Jobin-Yvon SPEX Fluorolog 3.22 spectrofluorimeter at 25 °C. The luminescence quantum yields were calculated with respect to quinine sulfate in 1 N H2SO4 (φ ) 0.546).26 The corresponding deareated solutions were purged with argon. The solutions for the Stern-Volmer experiments were degassed by a freeze-pump-thaw procedure (three cycles ×10-5 Torr). Fluorescence Decay Measurements. The samples were excited at 316 nm using a coaxial flash lamp (IBH, 5000 system) filled with nitrogen. The lamp pulses (∼1.2 ns fwhm) are monitored by a synchronization photomultiplier; the PM signal is shaped in a constant fraction discriminator (Canberra 2126) and directed to a time to amplitude converter (TAC, Canberra 2145) as start pulses. Emission wavelengths (350 and 390 nm) are selected by a monocromator (Oriel 77250) imaged in a fast photomultiplier (9814B Electron Tubes Inc.). The PM signal is shaped as before and delayed before entering the TAC as stop pulses. The analogue TAC signals are digitized (ADC, ND582) and stored in a multichannel analyzer installed in a PC (1024 channels with 0.076 ns/channel). The error in decay time determination (global error) was estimated to be ∼200 ps, by consecutive independent measurements (at least three).
TABLE 1: Electronic Absorption Data for Compounds 1 and 2 in 5 × 10-5 M Ethanol Solutions compound
absorption λmax, nm (10-3, M-1 cm-1)
1 2
286 (9.60), 317 (2.85), 330 (2.53) 287 (9.27), 316 (2.44), 332 (2.25)
The analysis of the decays is carried out with the method of modulating functions extended by global analysis as implemented by Striker.27 Molecular Modeling. Molecular models were produced with HYPERCHEM version 7.5. (Hypercube (2005) for illustration purposes only).28 Results and Discussion Absorption and emission spectra of compounds 1 and 2, carried out in air-equilibrated ethanol solutions at room temperature are shown in Figure 1. The absorption spectra of both isomers show typical π-π* transitions of the substituted naphthalene chromophore, with a maximum centered at ca. 285 nm and two well-defined vibrational bands at ca. 315 and 330 nm (Figure 1 and Table 1). The fluorescence emission of both isomers is clearly different from the emission of the naphthalene chromophore in dilute solution (see Figure S1, Supporting Information). In both cases, there is a lack of the typical resolution of the emission spectrum of the naphthalene. Moreover, they present a broad and redshifted fluorescence band (greater for compound 2), typical of excimeric emission.29 Compound 1 shows a maximum at 350 nm, while compound 2 shows a significantly higher Stokes shift, with a maximum at 390 nm. This difference could be related with differences in the superposition between the two naphthalene rings in both isomers. The geometry of compounds 1 and 2 was optimized using AM1-RHF semiempirical method.28 The obtained minimum energy configurations of the compounds are presented in Figure 2 and are in agreement with the X-ray crystal structures of the compounds, recently obtained by some of us.24,30 Although the syn arrangement of the two naphthalene units in 1 could have in principle ensured a better interaction between the aromatic rings, molecular modeling calculations suggest that only in compound 2 there is an effective superimposition of π-systems, namely, of two benzenic ring belonging to the two naphthalenic moieties in the most stable conformation of naphthalenophane 2. This fact could favor stronger π-π interactions between the two chromophoric groups of 2 and could be the reason for the reflected red shift in the emission spectra of this complex with respect to 1.
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Figure 2. Minimum-energy configuration of compounds 1 (left) and 2 (right): blue spheres, carbon atoms; red spheres, oxygen atoms.
TABLE 2: Quantum Yields and Lifetime Values for Compounds 1 and 2 in 5 × 10-7 M Ethanol Solutionsa compound quantum yield φ, air (argon) lifetime τ (ns), air (argon) 1 2 a
0.15 ( 0.02 (0.46 ( 0.05) 0.13 ( 0.02 (0.31 ( 0.04)
3.1 ( 0.2 (3.2 ( 0.3) 3.0 ( 0.3 (3.3 ( 0.3)
The values shown are averages of at least three measurements.
The fluorescence lifetimes and quantum yields of compounds 1 and 2 measured at room temperature, in air-equilibrated solutions and in solutions saturated with argon, are reported in Table 2. The decays of freshly prepared solutions show similar decay times for both compounds (ca. 3 ns), which are much shorter than those of naphthalene in ethanol (84 ns in degassed solutions and 21 ns in air equilibrated solutions, see Figures S2-S4 in the Supporting Information). The fluorescence quantum yields of both compounds decrease in the presence of oxygen (ca. 3-4 times decrease in air equilibrated solutions, see Table 2). However, the strong fluorescence quenching observed in air equilibrated solutions (2.1 × 10-3 M in dissolved oxygen) is not displayed in the fluorescence decays (Table 2), where only 3-9% reduction in the decay times is observed. Notice that this small reduction is compatible with the diffusional quenching by O2 (1 × 1010 to 3 × 1010 L mol-1s-1) and very close to that calculated for the naphthalene solutions (1.7 × 1010 L mol-1 s-1). The much larger quenching observed in the emission spectra is probably due to the existence of static quenching, which can be accounted for by the formation of a ground state CT complex with oxygen,23 whose lifetime is outside our time resolution. The geometry of the two isomers affects the rate of complex formation/dissociation with oxygen. Thus, while 30 min of bubbling with argon is necessary to attain a stable emission spectrum of compound 1 (maximum emission), compound 2 needs approximately 2 h for complete removal of the quenching by oxygen. On this basis the relative conformation of the latter compound seems to give a stronger and kinetically more stable complex with oxygen. Bubbling with argon could eventually promote the exchange of molecular oxygen with argon in the adduct, and this phenomenon could reasonably be affected by the different affinity of the compound for oxygen molecule. In order to identify possible argon associated effects, ethanol solutions (5 × 10-7 M) of the compounds were studied in different conditions: under degassed conditions (vacuum after three freeze-pump-thaw cycles), under oxygen-saturated atmosphere and under argon-saturated atmosphere. After the preliminary freeze-pump-thaw degassing procedure, different vacuumoxygen-vacuum-argon-vacuum cycles were performed. The results of these experiments are reported in Figure 3. It is apparent that only the presence of molecular oxygen affects the recorded emission intensity to a significant extent. The
Figure 3. Emission spectra of compound 2 in vacuum (solid lines), under oxygen atmosphere (dotted line), and under argon atmosphere (dashed line). λexc ) 316 nm.
changes are reversible and the emission intensity is restored after new freeze-pump-thaw cycles and/or in the presence of argon. There are two possible routes for the emission quenching by oxygen: the formation of a ground state association between the emissive molecule and the quencher (static quenching) and the diffusion of the emissive species in the excited state and its encounter with a quencher molecule that gives rise to its deactivation (dynamic quenching).31 The simultaneous operation of both quenching mechanism is accounted for by the Stern-Volmer equation (eq 1)
I0 ) (1 + kqτ0[Q])(1 + K′[Q]) I
(1)
where I0 and I are the intensities of the luminophore in the absence and presence of oxygen, kq is the quenching constant, τ0 is the lifetime of the unquenched species, and K′ is the equilibrium constant for the formation of the complex LQ between the luminophore and oxygen (eq 2). K′
L + Q y\z LQ
(2)
Figure 4. Changes in the emission spectra of a 5 × 10-7 M ethanol solution of compound 2 with different quantities of oxygen (0%, dasheddotted line; 21%, solid line; 54%, dashed line; 100%, dotted line).
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Figure 5. Stern-Volmer plots from the data recorded from stationary state (I0/I, filled circles) and transient state (τ0/τ, open circles) for compounds 1 (a) and 2 (b).
According to eq 1, when both types of quenching mechanisms operate, the plot of the I0/I ratio as a function of the quencher concentration (Stern-Volmer plot) is not linear. In order to quantify the relative weight of dynamic versus static quenching, 3 mL of 5 × 10-7 M ethanol solutions of 1 and 2 were saturated with different argon/oxygen mixtures of different composition, previously prepared with exactly known concentrations of oxygen in argon (0, 21, 54, and 100%). Changes observed in the emission spectra of compound 2 are illustrated in Figure 4. Similar results were obtained with compound 1. For the samples containing increasing amounts of oxygen, the fluorescence decay times were also recorded. Stern-Volmer plots of I0/I vs [O2] obtained for compound 1, from steady state intensities, are given in Figure 5 (filled circles), together with τ0/τ data obtained from fluorescence decay times (open circles). It is clearly seen from the plot of the stationary state intensities that a linear relation exists between I0/I and [O2], which implies that only one of the quenching mechanisms, static or dynamic, is operative or that the contribution of one of the two mechanisms is negligibly small in the explored concentration range of O2. From the time-resolved data, it is clear that the changes in the measured decay times are much less important than the quenching observed in steady state fluorescence. This means that either the CT complexes are nonemissive or they have decay times shorter than our time resolution (0.2 ns). Since static quenching is the predominant quenching mechanism in the steady state measurements, eq 1 can be reduced to eq 3
I0 ) 1 + K′[Q] I
(3)
and the value of the association constant K′ between 1 or 2 and molecular oxygen can be calculated. The obtained values are K′ ) 1320 ( 40 M-1 and 1120 ( 35 M-1 for 1 and 2, respectively. Similar experiments were carried out for compound 1 in acetonitrile and in chloroform, and as occurred in ethanol, the static mechanism dominates the quenching observed in the emission (less than 10% decrease in decay times against more than 4 times decrease in fluorescence emission for air equilibrated solutions). Association constants that are in the same order of magnitude (K′ ) 1700 ( 20 M-1 and K′ ) 1760 ( 30 M-1 in acetonitrile and chloroform, respectively) were obtained.
The structures of the CT complexes are not known. However the trapping of the molecular oxygen in the cavity could explain the unusually long times (∼2 h in the case of compound 2) needed to remove oxygen through argon bubbling. The values for the association constants obtained with our naphthalenophane compounds (∼103 M-1) are significantly higher than those obtained by Kenny and co-workers for naphthalene (∼50 M-1),23 which is also in agreement with the entrapment of the oxygen molecule in the naphtalenophane cavity. Conclusions The two atropisomeric naphthalenophanes 1 and 2 studied in this work show identical absorption spectra, while a red shift is observed in the emission spectrum of compound 2 with respect to 1. This is due to the excimeric origin of the emission and to the increased superposition between the aromatic rings in compound 2. The emission is strongly quenched in the presence of oxygen. The Stern-Volmer analysis comparing the quenching obtained from steady state and time-resolved data provides clear evidence of the formation of ground state charge transfer complexes between oxygen and the naphthalenophanes 1 and 2. This charge transfer between oxygen and aromatic molecules was previously observed by Kenny et al.23 The amount of dynamic quenching in aromatic molecules with long-lived luminescence, such as naphthalene or pyrene, is very high with respect to the static quenching component (CCT complex) and only small deviations were observed by Kenny et al., while in our case short-lived excimer emission makes dynamic quenching by oxygen inefficient and the static mechanism turns out to be dominant. The trapping of the molecular oxygen in the cavity of these compounds could explain the unusual long times needed to remove oxygen through argon bubbling and the significantly higher association constant values obtained with our naphthalenophane compounds (∼103 M-1) compared with those obtained by Kenny and co-workers for naphthalene (∼50 M-1). Acknowledgment. The authors are grateful to Fundac¸a˜o para a Cieˆncia e Tecnologia (Portugal) for financial support, PTDC/ QUI/67786/2006, and to MIUR PRIN 2006 (Italy). Authors also would like to thank Dr. Ana Isabel Aguiar Ricardo from the Departamento de Quı´mica, Universidade Nova de Lisboa, for the preparation of the oxygen-argon mixtures. Supporting Information Available: Emission spectra of naphthalene, compound 1, and compound 2 in ethanol and
Photophysical Study of Naphthalenophanes diluted solutions (Figure S1), decays of compounds 1 and 2 in ethanol without oxygen (Figure S2), decays of compounds 1 and 2 in ethanol with oxygen (21%) (Figure S3), and decay of naphthalene in ethanol with oxygen (21%) and without oxygen (Figure S4). This information is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Schweitzer, C.; Schmidt, R. Chem. ReV 2003, 103, 1685–1757. (2) Clark, L. C. US Patent 2,913,386, 1959. (3) Hitchman, M. C. Measurement of DissolVed Oxygen; Wiley: New York, 1978; p 130. (4) Skoog, D. A.; West, D. M.; Holler, F. J. Fundamentals of Analytical Chemistry; Saunders: Philadelphia, PA, 1988; p 344. (5) Zhujun, Z.; Seitz, W. R. Anal. Chem. 1986, 58, 220–222. (6) Baldini, F.; Bacci, M.; Cosi, F.; Del Bianco, A. Sens. Actuators, B 1992, 7, 752–757. (7) Freeman, T. M.; Seitz, W. R. Anal. Chem. 1981, 53, 98–102. (8) Nagl, S.; Baleiza˜o, C.; Borisov, S.; Scha¨ferling, M.; Wolfbeis, O. S.; Berberan-Santos, M. N. Angew. Chem., Int. Ed. 2007, 46, 2317–2319. (9) Baleiza˜o, C.; Nagl, S.; Scha¨ferling, M.; Berberan-Santos, M. N.; Wolfbeis, O. S. Anal. Chem. 2008, 80, 6449–6457. (10) Borisov, S. M.; Nuss, G.; Haas, W.; Saf, R.; Schmuck, M.; Klimant, I. J. Photochem. Photobiol., A 2009, 201, 128–135. (11) Wolfbeis, O. S. J. Mater. Chem. 2005, 15, 2657–2669. (12) Chu, C.-S.; Lo, Y.-L. Sens. Actuators, B 2008, 134, 711–717. (13) Jeong, M.; Nam, H.; Sohn, O.-J.; Rhee, J. I.; Kim, H. J.; Cho, C.W.; Lee, S. Inorg. Chem. Commun. 2008, 11, 97–100.
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