Langmuir 1997, 13, 6787-6794
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X-ray Photoelectron, UV-vis Absorption, and Luminescence Spectroscopic Studies of 2,2′-Cyanines Adsorbed onto Microcrystalline Cellulose A. M. Botelho do Rego,* L. Penedo Pereira, M. J. Reis, A. S. Oliveira, and L. F. Vieira Ferreira Centro de Quı´mica-Fı´sica MolecularsComplexo Interdisciplinar, Instituto Superior Te´ cnico, 1096 Lisboa Codex, Portugal Received May 12, 1997. In Final Form: September 30, 1997X 1,1′-Diethyl-2,2′-cyanine iodide and 1,1′-diethyl-2,2′-carbocyanine iodide were adsorbed onto microcrystalline cellulose by two different methods: by deposition from ethanolic solutions, followed by solvent evaporation, and also from ethanolic solutions in equilibrium with the powdered solid. Within experimental error, both methods provided the same fluorescence quantum yield of the adsorbed dyes in the concentration range 0.01-5.0 µmol of dye per gram of cellulose. Ethanol swells cellulose and some dye molecules become entrapped within the natural polymer chains and in close contact with the substrate. The use of dichloromethane, a solvent which does not swell microcrystalline cellulose, provides samples which exhibit a smaller fluorescence quantum yield. This is consistent with a larger degree of mobility (and also the formation of nonplanar and less emissive conformers) of the cyanines adsorbed on the surface of the solid substrate, while entrapment provides more rigid, planar, and emissive fluorophers. At the same time, the adsorption isotherms of 2,2′-cyanine on cellulose from alcoholic and dichloromethane solutions show that the specific cellulose surface area accessible for dye adsorption is larger when adsorption is from ethanol rather than from dichloromethane. For 2,2′-cyanine the fluorescence quantum yields (ΦF) determined were about 0.08 when dichloromethane (a solvent which does not swell cellulose) was used for sample preparation, while with ethanol ΦF was approximately 0.30. These values are about 3 orders of magnitude higher than those in solution, showing the importance of the rigid dry matrix in reducing the nonradiative pathways of deactivation of the (π, π*) first excited singlet state of this cyanine. X-ray photoelectron spectroscopic studies present evidence for hydrogen bonding of 2,2′-cyanine to cellulose at low loadings and for the formation of aggregates at higher loadings adsorbing from both ethanol and dichloromethane. This hydrogen bonding is assigned as involving dye molecules entrapped within the cellulose chains. On the other hand, for 2,2′-carbocyanine, evidence exists for an increase of hydrogen bonding with dye loading. This result together with evidence from ground-state diffuse reflectance absorption and luminescence is compatible with dye molecules being firmly bonded to the substrate by one of the nitrogen atoms, with the other unbound.
1. Introduction In the last few years some of us have concentrated our efforts on photochemical and photophysical studies of several probes, including cyanines, adsorbed onto an almost unexplored host: microcrystalline cellulose.1-5 From these studies, a large amount of information concerning room temperature fluorescence and phos* To whom correspondence should be addressed: Tel, 351-18419255; fax, 351-1-3536985; e-mail,
[email protected]. X Abstract published in Advance ACS Abstracts, November 15, 1997. (1) (a) Oliveira, A. S.; Vieira Ferreira, L. F.; Worrall, D.; Wilkinson, F. J. Chem. Soc., Faraday Trans. 1996, 92, 4809. (b) Vieira Ferreira, L. F.; Oliveira, A. S.; Worrall, D.; Wilkinson, F. J. Chem. Soc., Faraday Trans. 1996, 92, 1217. (c) Vieira Ferreira, L. F.; Oliveira, A. S.; Henbest, K.; Worrall, D.; Wilkinson, F. RAL Annu. Rep., in press. (d) Vieira Ferreira, L. F.; Netto-Ferreira, J. C.; Oliveira, A. S.; Costa, S. M. B. Bol. Soc. Port. Quı´m. 1996, 60, 50. (2) (a) Vieira Ferreira, L. F.; Netto-Ferreira, J. C.; Khmelinskii, I. V.; Garcia, A. R.; Costa, S. M. B. Langmuir 1995, 11, 231. (b) Vieira Ferreira, L. F.; Freixo, M. R.; Garcia, A. R.; Wilkinson, F. J. Chem. Soc., Faraday Trans. 1992, 88, 15. (c) Vieira Ferreira, L. F.; Garcia, A. R.; Freixo, M. R.; Costa, S. M. B. J. Chem. Soc., Faraday Trans. 1993, 89, 1937. (d) Netto-Ferreira, J. C.; Vieira Ferreira, L. F.; Costa, S. M. B. Quı´m. Nova 1996, 19, 230. (3) (a) Wilkinson, F.; Leicester, P.; Vieira Ferreira, L. F.; Freire, V. M. M. R. Photochem. Photobiol. 1991, 54, 599. (b) Wilkinson, F.; Kelly, G. P.; Vieira Ferreira, L. F.; Freire, V. M. M. R.; Ferreira, M. I. J. Chem. Soc., Faraday Trans. 1991, 87, 547. (4) (a) Vieira Ferreira, L. F.; Netto-Ferreira, J. C.; Costa, S. M. B. Spectrochim. Acta 1995, 51A, 1385. (b) Vieira Ferreira, L. F.; Oliveira, A. S.; Khmelinskii, I. V.; Costa, S. M. B. J. Lumin. 1994, 60 & 61, 485. (c) Wilkinson, F.; Worrall, D. R.; Vieira Ferreira, L. F. Spectrochim. Acta 1992, 48A, 135. (5) (a) Levin, P. P.; Vieira Ferreira, L. F.; Costa, S. M. B. Chem. Phys. Lett. 1990, 173, 277. (b) Levin, P. P.; Vieira Ferreira, L. F.; Costa, S. M. B. Langmuir 1993, 9, 1001.
S0743-7463(97)00490-3 CCC: $14.00
phorescence,1-4 triplet-triplet energy transfer,3 and electron and hydrogen atom transfer5,2a,d was obtained. All these processes are strongly dependent on the interaction of the probes with the matrix, which may provide a rigid environment profoundly affecting the properties of the guest molecules. Adsorption of probes in this solid substrate was achieved from polar protic (e.g., alcohols) or aprotic (e.g., acetonitrile or acetone) solutions. When microcrystalline cellulose is added to this solution, cellulose to cellulose hydrogen bonds are replaced by cellulose to solvent bonds and the matrix may exhibit a certain degree of swelling which depends on the solvent used for sample preparation.1,2 Probes can then penetrate into submicroscopic pores of the solid substrate and stay entrapped into the cellulose chains after solvent removal. As a result of entrapment, 1,1′-diethyl-2,2′-cyanine iodide (2,2′-cyanine) and 1,1′-diethyl-2,2′-carbocyanine iodide (2,2′-carbocyanine) exhibit fluorescence quantum
yields of 0.30 and 0.33, respectively, which are about 3 orders of magnitude greater than those observed in ethanolic solutions.1b,6 Similar results have been obtained for oxacarbo and oxadicarbocyanines adsorbed onto mi© 1997 American Chemical Society
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crocrystalline cellulose1a or when the chemical structure of the dye is rigid.7 The degree of dryness of cellulose was found to affect the aggregation of both dyes in the matrix and in this way it also affects the fluorescence emission. 2,2′-Cyanine forms both H (sandwich type) and J (head to tail) aggregates in air equilibrated, moisture containing cellulose samples; 2,2′-carbocyanine forms only H aggregates in wet cellulose.1b In both cases, the amount of aggregation is larger in wet samples when compared with dry cellulose, as revealed by ground-state diffuse-reflectance studies.1b X-ray photoelectron spectroscopy (XPS) is a powerful tool for surface characterization. The technique is strongly surface sensitive due to the photoelectron escape depth: only the first 10 to 20 atomic surface layers can be analyzed.8 XPS is capable of providing information about elemental concentration by angle-resolved analysis only for nearly ideal surfaces, i.e., those which are flat, smooth, nonporous, homogeneous in the plane of the surface, nonvolatile, conductive, and stable with respect to ultrahigh vacuum. Cellulose is far from fulfilling these criteria. However, in spite of these difficulties, a lot of interesting work with cellulose,8-10 cellulose-based materials,11-13 and other porous materials14,15 has recently been reported. In this paper, and following recent studies of 2,2′cyanines1b and several oxacyanines1a adsorbed onto microcrystalline cellulose,16 we report spectroscopic information regarding adsorbed organic species from two types of samples: In type I samples we eliminate the solvent in which the probe is dissolved from the solvent/cellulose mixture by simple evaporation. Final removal of solvent is performed in a vacuum chamber. With this procedure we avoid observing the spectrum of the probe species in solution (as in slurry samples where solution is in close contact with the adsorbent) which would interfere with the spectrum of the adsorbed species. However, forced vaporization of solvent leaves solute molecules on the surface of the powdered adsorbent and may in this way “force” aggregate formation, particularly when the initial solution concentration of the probe is high. Of course this “forced” aggregation depends on the specific surface area of the adsorbent, greater aggregation being seen with smaller specific surface areas. As a result of this procedure microcrystals of the probe can be formed on the surface of microcrystalline cellulose for high cyanine loadings. Type II samples are prepared from systems where the solution containing the probe and the powdered adsorbent (6) (a) Roth, N. J. L.; Craig, A. C. J. Phys. Chem. 1974, 78, 1154. (b) Treadwell, C. J.; Keary, C. M. Chem. Phys. 1979, 43, 307. (7) O’Brien, D. F.; Kelly, T. M.; Costa, L. F. Photogr. Sci. Eng. 1974, 18, 76. (8) Briggs, D.; Seah, M. S. Practical Surface Analysis by Auger and X-ray Photoelectron Spectroscopy; John Wiley & Sons, Ltd.: New York, 1983. (9) Sapieha, S.; Verreault, M.; Klemberg-Sapieha, J. E.; Sachier, E.; Wertheimer, M. R. Appl Surf. Sci. 1990, 44, 165. (10) Toth, A.; Faix, O.; Rachor, G.; Bertoti, I.; Szekely, T. Appl. Surf. Sci. 1993, 72, 209. (11) Istone, W. K. J. Vac. Sci. Technol. 1994, A 12, 2515. (12) (a) Kamdem, D. P.; Riedl, B.; Adnot, A.; Kaliaguine, S. J. Appl. Polym. Sci. 1991, 43, 1901. (b) Kamdem, D. P.; Riedl, B. Colloid Polym. Sci. 1991, 269, 595. (13) Gardner, D. J.; Generalla, N. C.; Gunnells, D. W.; Wolcott, M. P. Langmuir 1991, 7, 2498. (14) (a) Desaeger, M.; Reis, M. J.; Botelho do Rego, A. M.; Lopes da Silva, J. D.; Verpoest, I. J. Mater. Sci. 1996, 31, 6305. (b) Reis, M. J.; Botelho do Rego, A. M.; Lopes da Silva, J. D.; Soares, M. N. J. Mater. Sci. 1995, 30, 118. (15) (a) Bradley, R. H.; Sutherland, I.; Sheng, E. J. Colloid Interface Sci. 1996, 179, 561. (b) Garbassi, F.; Balducci, L.; Chiurlo, P.; Deiana, L. Appl. Surf. Sci. 1995, 84, 145. (16) Battista, O. A. Microcrystalline Cellulose. In Encyclopedia of Polymer Science and Technology; Mark, H. F., Gaylord, N. G., Bikales, N. M., Eds.; Wiley: New York 1965; Vol. 3, p 285.
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are in a dynamic equilibrium. Solute molecules are adsorbed onto adsorption sites and at the same time solutes may leave the adsorbent and redissolve into the solvent. This process goes on until an equilibrium is reached for each temperature and each solute concentration. The equilibrated solution is then removed by filtration. The existence of an equilibrium for adsorption allows us to determine the surface area of the substrate, by assuming a given adsorption model, as well as a surface area for planar cyanines and a planar conformation for the adsorbate. These two types of samples will be compared from the standpoint of UV-vis ground-state diffuse reflectance measurements as well as steady-state luminescence experiments, which together provide insight into the nature of the interactions between the cellulosic substrate and the probes. With the use of XPS and more specifically through the nitrogen 1s photoelectron region, conclusions regarding the cyanine molecule planarity, and intramolecular interactions, can be drawn. With these studies we hope to get some insight into the nature of the interactions between these cyanines and the polymer chains, particularly the extent of hydrogen bonding of the probe to the substrate which is of major importance in the immobilization process and in minimizing the nonradiative transitions of the excited states. 2. Experimental Section 2.1. Materials and Sample Preparation. 1,1′-Diethyl-2,2′cyanine iodide and 1,1′-diethyl-2,2′-carbocyanine iodide were purchased from Aldrich in the highest purity available. Both compounds were used as received after checking purity by the use of UV-vis absorption spectra and thin-layer chromatography. Dichloromethane was HPLC grade from Romil Chemicals. Ethanol was from Merck (Uvasol grade). All solvents were used as received, after checking purity by UV and visible absorption spectrophotometry. Molecular sieves (3 and 4 Å, 4-8 mesh, Aldrich, previously activated by slow heating to 250 °C under vacuum) were used in some cases for sample preparation when very dry solvents were needed. Microcrystalline cellulose16 (Fluka DSO) with 50 µm average particle size was dried under vacuum (ca. 10-3 mbar) at 60 °C for at least 24 h before sample preparation. Two main types of samples were prepared: Type I includes samples where the probe is deposited by solvent evaporation. In this case samples were prepared as described in refs 1a-c except for the solvent removal step which was performed in an acrylic chamber with two electrically heated shelves (Heto, Models FD 1.0-110 and FC-2R/H) with temperature control (40 ( 1 °C) and reduced pressure (ca. 10-3 mbar, g12 h). Type II includes samples where the powdered adsorbent is in equilibrium with a solution of the probe, the equilibrated solution being removed by filtration. In both cases, two different solvents were used for adsorption, ethanol, and dichloromethane, producing samples here called ethanolic samples and dichloromethane samples, respectively. For type II samples, room temperature (∼20 °C) adsorption isotherms of 2,2′-cyanine on microcrystalline cellulose were obtained. Solutions were kept in contact with cellulose for a period of 3 days after vigorous agitation in an ultrasonic bath. After this period suspensions were filtered (the first filtrate was rejected to avoid concentration changes by the dye adsorption to the filter paper) and analyzed by absorption spectrophotometry. For comparison purposes, other samples were prepared by mechanically mixing well ground 2,2′-cyanine and 2,2′-carbocyanine crystals with microcrystalline cellulose powder, using an agate pestle and mortar. As will be discussed later, different solvents produce different degrees of swelling of microcrystalline cellulose, enabling different degrees of penetration and entrapment of the probes into the cellulosic polymer chains. This effect has already been described
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by us for benzophenone2a and is now further detailed for these two cyanines adsorbed onto microcrystalline cellulose (see Results and Discussion). 2.2. Ground-State Absorption Spectra in the UV-vis Regions and Steady-State Emission Experiments. Groundstate absorption studies of 2,2′-cyanine and 2,2′-carbocyanine adsorbed onto microcrystalline cellulose were performed using an OLIS 14 UV-vis/NIR spectrophotometer with a diffuse reflectance attachment. The integrating sphere is 90 mm in diameter and internally coated with a standard white coating. The standard apparatus was modified to include the possibility of using short-wave-pass filters (Corion 550-S and 600-S) which exclude luminescence from the two cyanines from reaching the detector (Hamamatsu, Model R955). Further experimental details and a description of the system calibration used to obtain accurate reflectance measurements are given in refs 1 and 2. Solution measurements were made using the same apparatus in the transmission mode. Corrected steady-state fluorescence and phosphorescence emission and excitation spectra of the cyanines under study adsorbed onto microcrystalline cellulose were obtained using a home-made fluorometer previously described.17 2.3. XPS Experiments. For XPS studies, samples of dry cellulose with adsorbed dye were moulded as for IR experiments, without any solid solvent, and held to the sample holder stub with a tungsten spring. Samples were analyzed by X-ray photoelectron spectroscopy (XPS) using a XSAM800 (KRATOS) X-ray spectrometer operated in the fixed analyzer transmission (FAT) mode, with a pass energy of 20 eV and the nonmonochromatized Mg KR X-radiation (hυ ) 1253.7 eV). Typical operating parameters were 13 kV and 10 mA. Samples were analyzed in ultrahigh vacuum (UHV), and typical base pressure in the sample chamber was in the range of 10-7 Pa. All sample transfers were made in air. Samples were analyzed at room temperature, at a take-off angle (TOA) of 90°. Spectra were collected and stored in 200 channels with a step of 0.1 eV using a Sun SPARC Station 4 with Vision software (Kratos). The curve fitting for component peaks was carried out with a nonlinear least-squares algorithm using a mixture of Gaussian and Lorentzian peak shapes.
3. Results and Discussion 3.1. Liquid Phase Adsorption Isotherms. The adsorption isotherm of the 2,2′-cyanine on cellulose from ethanolic solutions (Figure 1, full squares) is compatible with the Langmuir adsorption model, i.e., the variation of the number N of adsorbed moles, per gram of cellulose, with the dye concentration in equilibrated solutions, ceq, follows the equation18
Kceq N ) Nm 1 + Kceq
(1)
ceq ceq 1 + ) N KNm Nm
(2)
or, in a linear form
where Nm and K are the number of molecules of the saturated monolayer and the “adsorption equilibrium constant”, respectively. By least-squares linear regression, the adjusted parameters Nm and K were (3.0 ( 0.3) × 10-6 mol per gram of cellulose and (3.0 ( 0.8) × 104 L mol-1. Assuming an (17) (a) Vieira Ferreira, L. F.; Costa, S. M. B.; Pereira, E. J. J. Photochem. Photobiol., A 1991, 55, 361. (b) Vieira Ferreira, L. F.; Costa, S. M. B. J. Lumin. 1991, 48 & 49, 395. (18) Adamson, A. W. Physical Chemistry of Surfaces, 5th ed.; John Wiley & Sons, Inc.: New York, 1990; Chapter 11.
Figure 1. (a) Adsorption isotherm of 2,2′-cyanine on cellulose from solutions: full lines, Langmuir isotherm calculated by the use of the parameters obtained with a linear least-squares regression of c/N versus the equilibrium concentration; dashed lines, curves corresponding to the parameters affected by the fitting statistical errors values; (9) experimental points for ethanolic solutions; (0) experimental points for dichloromethane solutions. (b) Langmuir linearization. In both part a and b thicker lines correspond to dicloromethane solutions.
area of influence of 133 Å2 for the 2,2′-cyanine molecule,19 the specific surface area accessible to the dye, A, is 2.40 ( 0.25 m2 per gram of cellulose. The same isotherm obtained replacing ethanol with dichloromethane is also presented in Figure 1 by empty squares. Assuming the same area of influence as for the ethanolic solution, least-squares regression yields Nm ) (1.44 ( 0.12) × 10-6 mol per gram of cellulose, K ) (3.8 ( 2.8) × 104 L mol-1, and A ) 1.15 ( 0.10 m2 per gram of cellulose. The relative error associated with K is larger than that obtained in the preceding case due to the lower number of experimental points. Even so, it is possible to conclude that the adsorption energetics (related to K) have the same order of magnitude in both systems. The cellulose surface area accessible to the dye when adsorbed from ethanol is larger than the one obtained when adsorption is from dichloromethane. This result is fully consistent both with the swelling effect of ethanol and with the incapacity of dichloromethane for breaking cellulose to cellulose hydrogen bonds between the polymer chains, previously reported using benzophenone as a probe.2a The dispersion of the data is related with the use of volatile solvents which evaporate during filtration. The Langmuir isotherm provides parameters which have physical meaning and is suitable for this case since we did not find evidence for multilayer adsorption. The use of “more realistic” isotherms (Freundlich’s, as an example) is not appropriate due to the fact that it does not provide parameters with physical meaning. (19) (a) Padday, J. F.; Wickham, R. S. Trans. Faraday Soc. 1965, 62, 1283. (b) Padday, J. F. Trans. Faraday Soc. 1964, 60, 1325.
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carbocyanine. At wavelengths shorter than 350 nm, the absorption of the dye is masked by the intrinsic absorption of microcrystalline cellulose.1,2 At high loadings (g0.5 µmol of cyanine per gram of cellulose) 2,2′-carbocyanine samples prepared from ethanolic solution show the formation of “sandwich type” aggregates, as Figure 2b clearly shows. Curve number four in the same figure suggests that either aggregate formation is higher for dichloromethane samples than for ethanolic samples or, alternatively, the broadening effect shown in curve 4 of Figure 3b (which may be compared with curve 3 for ethanolic samples) may be due to different conformations of 2,2′-carbocyanine on the surface.20 Higher loadings show the same behavior as curve 4. The remission function F(R) was obtained by calculating the Kubelka-Munk function for optically thick samples, i.e., those where any further increase in thickness does not affect the experimentally determined reflectance R2b,21
F(R) ) (1 - R)2/(2R) ) K/S
(3)
K and S are the absorption and scattering coefficients with dimensions (distance)-1. For an ideal diffuser, where the radiation has the same intensity in all directions, K ) 2C ( is the Naperian absorption coefficient, C is the concentration).21 Since the substrate usually absorbs at the excitation wavelength λe
F(R)probe ) F(R)total - F(R)cell )
Figure 2. (a) Reflectance spectra of 1,1′-diethyl-2,2′-cyanine adsorbed onto microcrystalline cellulose: curve 1 is the blank; curve 2, 0.1 µmol g-1, “type I” sample from ethanol; curve 3, 4.2 µmol g-1, “type I” sample from dichloromethane; curve 4, 5.0 µmol g-1, “type I” sample from ethanol; curve 5, 4.2 µmol g-1, “type II” sample from dichloromethane. (b) Reflectance spectra of 1,1′-diethyl-2,2′-carbocyanine adsorbed onto microcrystalline cellulose: curve 1 is the blank; curve 2, 0.1 µmol g-1; curve 3, 1.0 µmol g-1, sample from ethanol; curve 4, 1.0 µmol g-1 sample from dichloromethane; curve 5, 5.0 µmol g-1 sample from ethanol. All samples are “type I”. (c) Remission function values for 1,1′-diethyl-2,2′-carbocyanine adsorbed onto microcrystalline cellulose normalized at the maximum of the remission function: curve 1 is 4.2 µmol g-1, “type I” sample from dichloromethane; curve 2 is 5.0 µmol g-1, “type I” sample from ethanol.
For 2,2′-carbocyanine, the number of moles needed for monolayer formation, Nm, should obviously be smaller for geometrical reasons. However, due to the incertitude in K, no thermodynamical comparison was possible with the 2,2′-cyanine case; for these reasons, these adsorption isotherms were not done. 3.2. Ground-State Absorption Spectra of 2,2′Cyanines Adsorbed onto Microcrystalline Cellulose. Figure 2a shows the spectral reflectance (R) of the dye versus wavelength for 2,2′-cyanine adsorbed onto microcrystalline cellulose while Figure 2b shows the same curves for 2,2′-carbocyanine. In both cases, solutions of different concentrations and in different solvents were used for sample preparation. The curves represent the percentage reflectance as a function of wavelength in the range 350700 nm for 2,2′-cyanine and 400-750 nm for 2,2′-
∑i2iCi/S
(4)
where F(R)cell is the blank obtained with a cell containing only microcrystalline cellulose. This equation predicts a linear relation for the remission function of the probe as a function of concentration (for a scattering coefficient independent of wavelength) whenever the probe is in the form of a monomer. Figure 2c shows the remission function F(R) for 5 µmol g-1 2,2′-carbocyanine samples adsorbed on cellulose from ethanol and dichloromethane which clearly shows a broad band in the 450-550 nm range in the latter case. This is consistent with the existence of different conformers in this case coexisting with aggregate formation. Again, all these facts are consistent with ethanol being a good swelling solvent for microcrystalline cellulose while dichloromethane is not. Figure 2a gives evidence for a different behavior for 2,2′-cyanine. Type II (or “equilibrium” samples, curve 5) do not exhibit head to tail dimers (J aggregates) while type I samples (“forced deposition”) do, with an absorption peaking at about 575 nm. In this case, for a given number of moles of cyanine per gram of adsorbent, the amount of J aggregates formed with dichloromethane is higher than for ethanolic samples. This result, again, is consistent with a larger specific substrate surface area in the case of samples prepared from ethanolic solution. 3.3. Luminescence Studies of 2,2′-Cyanines and 2,2′-Carbocyanine Adsorbed onto Microcrystalline Cellulose. We recently reported fluorescence emission studies of 2,2′-cyanines and 2,2′-carbocyanines at room temperature when these dyes were adsorbed onto microcrystalline cellulose1b and when the solvent used for sample preparation was ethanol. No significant phos(20) Sturmer, D. M. In Kirk Othmer Encyclopedia of Chemical Technology, 4th ed.; Interscience Publishers, John Wiley & Sons: New York, 1994; Vol. 7, p 782. (21) Wilkinson, F.; Kelly, G. P. Handbook of Organic Photochemistry; Scaiano, J. C., Ed.; CRC Press: Boca Raton, FL, 1989; Vol. 1, p 293, and references therein.
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Figure 3. (a) Variation of the intensity of fluorescence of 1,1′diethyl-2,2′-cyanine adsorbed onto microcrystalline cellulose (steady-state) measured as the total area under the corrected emission spectrum IF, as a function of the square root of the concentration of dye adsorbed onto microcrystalline cellulose. The solvents used for sample preparation were (0) ethanol and (9) dichloromethane. Full or open squares are used for type I samples and crosses for type II samples. (b) Same data as in part a but now for 1,1′-diethyl-2,2′-carbocyanine.
phorescence as compared to fluorescence emission was detected for any of the samples at room temperature when exciting at 337 nm with a nitrogen laser. According to eq 5
IF ) CI0(1 - Rλe)fdyeφF
(5)
a correlation of fluorescence emission intensity (IF), measured as the total area under the emission spectra as a function of the light absorbed by the dye at the excitation wavelength, with (1 - Rλe)fdye, should be observed1b and was found to be the case for both cyanines. In eq 5, C is a geometrical factor, I0 is the excitation intensity at the excitation wavelength, λe, Rλe is the reflectance measured at the excitation wavelength, fdye is the fraction of the excitation light absorbed by the dye at the excitation wavelength λe, and φF is the fluorescence quantum yield. Alternatively IF versus square root of concentration of the dye is also linear for small fractions of absorbed light. Deviations from linearity occur for higher loadings due to H and J aggregate formation, which are not fluorescent. Figure 3a shows the variation of intensity of fluorescence of 2,2′-cyanine as a function of the square root of the concentration of dye adsorbed onto microcrystalline cellulose for type I and type II samples from ethanol and also from dichloromethane. As reported before,2b fluorescence quantum yields of probes adsorbed onto powdered solids can be determined by the use of eq 6
φFu ) φFsIFu(λe)(1 - Rsλe)f sIos(λe)/ IFs(λe)(1 - Ruλe)f uIou(λe) (6) where the superscripts u and s refer to the unknown and standard samples. Ios(λe)/Iou(λe) can be easily obtained provided the energy profile of the system is accurately determined.17 Rhodamine 101 was used as a reference
compound (φFs ) 1.0). The reasons for that were previously reported in refs 2b and 2c. It is important to emphasize several features: First, the initial slope (from the initial linear region) for dichloromethane samples (φF ∼ 0.1) is smaller than for ethanol samples (φF ∼ 0.3). Fluorescence emission intensity from type I and type II samples is the same, within experimental error, as is clearly shown in Figure 3a. However the maximum of the curve is reached for smaller dye concentrations in the case of dichloromethane, and this is true for both dyes. Additionally, aggregation begins at lower dye loadings for the largest molecule, i.e., carbocyanine. These findings are once more consistent with the previously described picture: the use of ethanol, a polar and protic solvent, as a swelling agent for microcrystalline cellulose allowed us to entrap 2,2′-cyanine and 2,2′carbocyanine into the polymer chains, providing a rigid environment capable of enhancing fluorescence from these dyes. With the use of dichloromethane, a nonprotic and slightly polar solvent, a smaller cellulosic specific surface area is available for adsorption. As a consequence, aggregation increases in the latter case. At the same time entrapment provides increased rigidity of the dye while attachment of cyanines to the cellulose-free surface allows the probe a larger degree of movement and consequently increases the nonradiative pathways of deactivation. Figure 3 suggests that the fluorescence quantum yields for both dyes are identical for type I samples prepared from ethanol. These findings are consistent with the observation of H and J aggregates in 2,2′-cyanines and only H aggregates in 2,2′-carbocyanines, simply because the quantum yields are determined from the initial slopes of φF versus (1 - Rλe)fdye and reflect the behavior of the monomers and not that of aggregated species which are formed at higher concentrations. As reported before,1a for air-equilibrated samples of oxacyanines adsorbed onto microcrystalline cellulose containing some moisture, aggregation becomes more important. In another case previously reported2c (Auramine O), the fluorescence quantum yield was strongly dependent on the degree of humidity of the sample. In spite of this, for these two 2,2′-cyanines, moisture effect is not so important in what concerns the fluorescence emission quantum yields due to the following facts: First, fluorescence quantum yields are determined for low loadings of the probe, i.e., for monomers. Second, removal of water increases rigidity of the substrate and this should point to an increase of φF. However in some cases, planarity is lost, thus reducing luminescence. The final emission certainly depends on a balance of these two tendencies. Figure 4 shows fluorescence spectra from type I and type II samples of 2,2′-cyanines when the solvent used for sample preparation was dichloromethane. As one can see comparing these spectra with previously reported data for the same dye adsorbed from ethanol (type I samples),1a the fluorescence emission is very similar. The important feature in curves 2 and 4 in Figure 4 is the significant self-absorption induced by J aggregates which strongly distorts the emission spectra. This distortion is clearly attributable to self-absorption and not due to emission from another species. 3.4. XPS Studies of 2,2′-Cyanines and 2,2′-Carbocyanine Adsorbed onto Microcrystalline Cellulose. The XPS study of adsorbed cyanines on cellulose was centered on the nitrogen region, as nitrogen is exclusively in the dye molecule whereas carbon is also a major constituent of the substrate. Regarding quantifi-
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Figure 4. Room temperature corrected fluorescence emission spectra of 1,1′-diethyl-2,2′-cyanine adsorbed onto microcrystalline cellulose excited at 490 nm. The concentrations are curves 1 and 2 1.0 µmol/g and curves 3 and 4 5.0 µmol/g. The solvent used for sample preparation was dichloromethane. Curves 2 and 4 are type I samples and curves 1 and 3 are type II samples.
cation of the amount of adsorbed cyanine, we used the ratio N/O instead of the ratio N/C for the same reasonsnitrogen exists exclusively in the dye and oxygen exists exclusively in the substrate. Another important feature may also be the existence of a contaminant, due to the fact that, even in UHV conditions, it is very difficult to assure a complete cleanliness of the surface. Contaminants are, in principle, mainly composed of carbon. The study of the N 1s region has other advantages: in principle, there is a relation between the form of the peak and the planarity of the molecule; for a completely planar molecule, there should be an equivalent electronic distribution around the two nitrogen atoms22si.e., a single N 1s photoelectron binding energyswhereas in a slightly distorted conformer the two nitrogen atoms are not equivalent since there exists a break in the conjugationsi.e., they should have different binding energies and hence a broadening of the N 1s peak should occur. The neighborhood of a nitrogen atom also has an effect on the electron density around it and, in this way, it affects the N 1s binding energy. As an example, if a nitrogen atom is involved in strong hydrogen bonding, its electron density will be lowered since electrons will be partially shared with more electronegative atoms (nitrogen electrons are shared with the hydrogen atoms which are electron deficient due to its bond to oxygen, a very electronegative atom) and, as a consequence, the N 1s binding energy increases. Large chemical shifts attributable to this were recently reported.23 Cyanine/Cellulose System. Figure 5 shows the XPS spectra for the N 1s region of several samples of 2,2′cyanine adsorbed onto microcrystalline cellulose from an ethanolic solution after total solvent evaporation. We can clearly see that the N 1s peak has a main component centered at a binding energy of 399.9 ( 0.1 eV for all concentrations under study. We assign this component to the nitrogen in a molecule with a planar configuration free from interactions apart from those with other dye molecules, as occur in H aggregates. Further support for this assignment was provided by the XPS analysis of thick films (∼10 monolayers) of 2,2′-cyanine deposited onto silicon wafers as well as mechanical mixtures of 2,2′-cyanine and cellulose containing dye loadings equivalent to 1.0 and 5.0 µmol g-1. The peak at higher binding energies (∼405 eV), (22) Griffiths, J. Colour and Constitution of Organic Molecules; Academic Press: London, 1976; Chapter 8, p 240. (23) Kerber, S. J.; Bruckner, J. J.; Wozniak, K.; Seal, S.; Hardcastle, S.; Barr, T. L. J. Vac. Sci. Technol., A 1996, 14, 1314.
Botelho do Rego et al.
Figure 5. X-ray photoelectron spectra of the N 1s region for three samples of 2,2′-cyanine adsorbed on cellulose from alcoholic solutions and allowing for the complete solvent evaporation: curve a, 0.1; curve b, 1.0; curve c, 10.0 µmol of dye per gram of cellulose.
usually associated with nitrogen bonded to highly electronegative atoms,24 is here assigned to strongly hydrogen bonded nitrogen. Its intensity decreases with increasing dye loading. This fact suggests that at low loadings a high percentage of molecules are intimately surrounded by substrate hydroxyl groups (entrapped) and also that with increasing dye loading that percentage decreases. At lower binding energies, another new component develops as dye concentration increases, indicating that an increasing fraction of molecules has a larger electron density near the nitrogen atoms (on both or at least on one of them). The assignment of this component is rather difficult. However, by comparison with results from optical absorption in the visible region we can say that it exists whenever J aggregates are present; one possibility therefore is to assign that component as arising from this kind of aggregate. An alternative explanation could be the existence of nonplanar conformers in which the loss of symmetry of the molecule causes the appearance of different electron densities around the nitrogen atom: one strongly bonded to the substrate and the other free above the surface. On the other hand, previous studies of cyanine films covering silicon wafers showed that, under the action of X-ray radiation, a slow degradation of dye occurs, displacing the N 1s peak toward lower binding energies. Taking into account that all spectra have been recorded, for all dye loadings, under the same conditions of irradiation and for the same time period, we exclude this hypothesis as a possible explanation for the different features in the spectra. In all samples where the cyanine was adsorbed to cellulose from dichloromethane solution, the same effects as for ethanolic samples were observed. However, some quantitative differences exist: (i) the low binding energy component appears for lower dye loadings. This fact also supports the assignment of this component to the existence of J aggregates which begins at lower concentrations in the dichloromethane case; (ii) the high binding energy component is less intense and centered at energies slightly lower than those in the ethanolic case. A third set of samples prepared by mechanically mixing cyanine with cellulose was studied. In this case, no intimate contact between the dye and the substrate exists: dye is mainly in the form of tiny crystals. We can see (Figure 6, curve a) that the N 1s peak becomes narrower, but centered at a higher binding energy (∼400.4 eV), than the central component of the samples prepared (24) E.g.: Wagner, C. D. NIST X-ray Photoelectron Spectroscopy Database, U.S. Dep. of Commerce, NIST, 1989.
Cyanines on Cellulose
Figure 6. X-ray photoelectron spectra of the N 1s region for three samples of 2,2′-cyanine adsorbed on cellulose in three different conditions: a) mechanical mixture; b) from dichloromethane solutions and allowing for complete solvent evaporation; (c) from alcoholic solutions and allowing for complete solvent evaporation. The concentration in all samples is 5.0 µmol of dye per gram of cellulose.
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Figure 8. X-ray photoelectron spectra of the N 1s region for three samples of 2,2′- carbocyanine adsorbed on cellulose in three different conditions: (a) mechanical mixture; (b) from dichloromethane solutions and allowing for the complete solvent evaporation; (c) from alcoholic solutions and allowing for the complete solvent evaporation. The concentration in all samples is 10.0 µmol of dye per gram of cellulose except for the dichloromethane case where it is 5.0 µmol g-1. Table 1. Atomic Ratio Nitrogen/Oxygen for Samples of 2,2′-Cyanine: Adsorbed on Cellulose from Ethanolic Solutions, from Dichloromethane Solution, and Mechanically Mixed
ethanol dichloromethane mechanical mixture
Figure 7. X-ray photoelectron spectra of the N 1s region for three samples of 2,2′-carbocyanine adsorbed on cellulose from alcoholic solutions and allowing for the complete solvent evaporation: curve a, 0.1; curve b, 1.0; curve c, 10.0 µmol of dye per gram of cellulose.
from the dissolved dye. This fact is attributed to a change of the electron density around the nitrogen atoms when the dye molecule is included within a microcrystal. Carbocyanine/Cellulose System. Carbocyanine concentration also has an effect over the spectral shape: Figure 7 displays the XPS N 1s region for three type I samples containing 0.1, 1.0, and 10.0 µmol of dye per gram of cellulose adsorbed from ethanolic solutions. This figure shows spectra which establish a clear difference between this case and the 2,2′-cyanine case: here with increasing dye loading, the high-energy component increases. Taking into account that this molecule is larger than the 2,2′-cyanine molecule and the number of possible conformers increases, as well as the fact that it exhibits H aggregation (see Figure 3) at high loadings, the following picture can be drawn: For low concentrations the interaction of entrapped dye molecules with cellulose is less effective than in the 2,2′-cyanine case. For higher loadings the fraction of nitrogen atoms strongly interacting with cellulose increases and this behavior is reflected by the high energy peak area. For some of the adsorbed molecules both nitrogens are involved in this interaction, the molecule being firmly attached to the substrate and possibly in a nonplanar conformation because of stereochemical constraints imposed by the hydrogen bonding. In other cases one of the nitrogen atoms may not be in contact with the surface and photoisomerization may occur, which accounts for the fluorescence quenching observed in samples with higher loadings. The comparison of samples where the dye was adsorbed from two different solvents and also mechanically mixed
0.1 µmol g-1
1 µmol g-1
5 µmol g-1
5.9 × 10-3 4.1 × 10-3
5.2 × 10-3 5.7 × 10-3
5.0 × 10-3
4.1 × 10-3
10 µmol g-1 5.3 × 10-3 10.4 × 10-3
with cellulose is informative regarding the conformation of the dye on the substrate (see Figure 8). The dye is present at a concentration of 10 µmol g-1 in both the mechanical mixture and the ethanolic case, whereas the concentration is 5 µmol g-1 in the dichloromethane case. Clearly, in this case only ethanolic and dichloromethane samples exhibit a strong interaction of the nitrogen atom with the substrate which obviously could not exist in the mechanical mixture. The main conclusion which emerges from Figures 7 and 8 is that the 2,2′-carbocyanine nitrogen atoms are not equivalent as they are in 2,2′-cyanine. This fact is consistent with a greater mobility of these probes, and this facilitates photoisomerization. As a consequence photoisomer emission may occur after absorption of a second photon (absorption of the first photon causes photoisomerization, and the second photon excites this prepared photoisomer). These facts are consistent with the “new emission” of cyanines adsorbed onto microcrystalline cellulose recently reported.1a-c The data provided by XPS studies point out that the interaction of 2,2′-cyanine with cellulose is quite different than that of 2,2′-carbocyanine, in agreement with a significant photoisomerization in the latter and not in the former. These findings support both the luminescence studies reported in ref 1b (“new emission” for 2,2′carbocyanine) and also the new absorption and luminescence data presented here for 2,2′-cyanine and 2,2′carbocyanine, i.e., the comparison of type I and type II samples and also those prepared from dichloromethane solutions. XPS studies also allow for the elemental quantification of the surface. Results are summarized in Table 1 for 2,2′-cyanine and in Table 2 for 2,2′-carbocyanine. These results clearly indicate the following: (i) 2,2′-cyanine molecules adsorbed onto cellulose from both solvents occupy deep sites, most probably pores. This explains the
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Table 2. Atomic Ratio Nitrogen/Oxygen for Three Samples of 2,2′-Carbocyanine: Adsorbed on Cellulose from Ethanolic Solutions, from Dichloromethane Solution, and Mechanically Mixed
ethanol dichloromethane mechanical mixture
0.1 µmol g-1
1 µmol g-1
5 µmol g-1
3.8 × 10-3 3.3 × 10-3
4.1 × 10-3 9.6 × 10-3
11.3 × 10-3
3.4 × 10-3
10 µmol g-1 9.4 × 10-3 7.5 × 10-3
invariance, within the experimental error, of the atomic nitrogen/oxygen (N/O) ratio for these samples in contrast with the mechanical mixture. In this case the absence of a close dye-substrate contact precludes that type of occupancy. (ii) In the case of 2,2′-carbocyanine there is a different behavior for ethanolic and dichloromethane samples: in the ethanolic case the invariance of the N/O ratio breaks down for the highest dye loading (10 µmol g-1) whereas for dichloromethane samples the invariance disappears or it breaks down for much lower dye loadings. This points to a generally greater difficulty for this molecule in occupying deep sites in the substrate as compared with the smaller 2,2′-cyanine molecule. The difference between the two kinds of samples is consistent with the fact that ethanol is a good swelling solvent. 4. Conclusions X-ray photoelectron spectroscopy and UV-vis absorption and luminescence studies have proved to be complementary techniques in the study of cyanine dyes adsorbed onto a natural polymer, microcrystalline cellulose. This allowed us to establish a clear picture of the specific
interactions of 2,2′-cyanine and 2,2′-carbocyanine with the substrate. The surface area accessible to adsorption is larger when ethanol is used for sample preparation due to the fact that this solvent swells microcrystalline cellulose and, in this way, creates new submicroscopic pores accessible to adsorption. This swelling effect does not occur with the nonprotic and slightly polar solvent, dichloromethane. A Langmuir type isotherm analysis provided specific surface areas of 2.40 ( 0.25 m2 and 1.15 ( 0.10 m2 per gram of cellulose for ethanol and dichloromethane, respectively. UV-vis ground-state diffuse reflectance absorption and luminescence studies showed a huge increase of fluorescence emission quantum yield when compared to solution studies for both cyanines, when these two probes are entrapped into microcrystalline cellulose, due to the reduction of mobility and formation of planar and emissive conformers, and to the amount and type of aggregates which are formed in each case. The emission depends on the solvent used to adsorb the probes. A complementary view is given by X-ray photoelectron studies which provides evidence for hydrogen bond formation as well as preferential attachment by one or two nitrogen atoms per cyanine molecule to the hydroxyl groups in the substrate. Acknowledgment. We acknowledge M. I. Morais for her technical support in sample preparation for XPS analysis and Dr. David Worrall for his careful revision of the English text. A. S. Oliveira thanks JNICT for her Ph.D. grant. This work was financed by Project Praxis/2/2.1/QUI/22/94. LA970490H
