Aggregation of Pseudoisocyanine Iodide in Cellulose Acetate Films

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Aggregation of Pseudoisocyanine Iodide in Cellulose Acetate Films: Structural Characterization by FTIR L. M. Ilharco* and R. Brito de Barros Centro de Quı´mica-Fı´sica Molecular, Complexo I, Instituto Superior Te´ cnico, Av. Rovisco Pais, 1049-001 Lisboa, Portugal Received April 18, 2000. In Final Form: July 31, 2000 Cellulose acetate films dyed with pseudoisocyanine iodide (1,1’-diethyl-2,2’-cyanine iodide) have been produced by spin coating and their structures characterized by FTIR spectroscopy. The aggregation of the cyanine during the spinning process was induced by addition of KI to the precursor solution, and the formation of J-aggregates was observed by UV-vis spectroscopy. The detailed analysis of the O-H stretching mode of cellulose acetate allowed us to understand the types of hydrogen bonds existing in the pure matrix films, in films containing just cyanine monomers and J-aggregates as well. It has been shown that cyanine monomers, even in a large concentration, have a small influence on the cellulose acetate structure, by favoring the replacement of some intermolecular by intramolecular hydrogen bonds. On the contrary, the presence of cyanine J-aggregates remarkably modifies the arrangement of the polymer chains, inducing an extensive formation of intermolecular hydrogen bonds in the C2, C3, and/or C6 positions of the glucopyranose rings. These intermolecular bonds do not involve the carbonyl groups, as the CdO stretching mode is not affected. This effect has been interpreted in terms of a higher degree of packing of the cellulose acetate amorphous phase, due to the presence of J-aggregates. The infrared spectra of the cyanine, in the wavenumber windows where the matrix does not absorb, have shown that the pseudoisocyanine molecule keeps the all-trans conformation upon aggregation. The modifications in the quinolines out of plane C-H deformations have indicated that the J-aggregates are formed by overlap of the phenyl rings of neighbor cyanines, leaving the heterocyclic moieties unperturbed.

Introduction Due to the restricted sensitivity of silver halide crystals to the ultraviolet, violet, and blue regions of the spectrum,1 the all-colored photographic processes need the application of dyes, such as cyanines. They have been used as spectral sensitizers over the last four decades.2,3 The association of cyanine molecules in solution leads to the formation of two types of aggregates, denoted by H and J. The latter were named after Jelley, who observed them for the first time.4,5 H-aggregates present, in the UV-vis absorption spectrum, a band at shorter wavelengths (∼480 nm) compared to the monomer maximum (∼525 nm), while J-aggregates are responsible for the rising of a sharp and intense band at longer wavelengths (∼580 nm).3,6,7 H-aggregates correspond to the association of just a few monomers,2 whereas J-aggregates may contain up to 105 molecules, with excitonic delocalization over a few tens of monomers.8 Their structure has been on focus since the first observation, as it may be of great help to understand the changes associated with the transition of the dye to a crystalline state. The initially proposed “head-to-tail” arrangement has developed to more detailed architectures that include the “staircase”, * Corresponding author. Tel: 351-21-8419220. Fax: 351-218464455. E-mail: [email protected]. (1) Peng, Z.; Zhou, X.; Carrol, S.; Geise, H. J.; Peng, B.; Dommisse, R.; Esmans, E.; Carleer, R. J. Mater. Chem. 1996, 6, 1325. (2) Emerson, E. S.; Conlin, M. A.; Rosenoff, A. E.; Norland, K. S.; Rodriguez, H.; Chine, D.; Bird, G. R. J. Phys. Chem. 1967, 71, 2396. (3) Herz, A. H. Adv. Coloid. Interface Sci. 1977, 8, 237. (4) Jelley, E. E. Nature 1936, 138, 1009. (5) Jelley, E. E. Nature 1937, 139, 631. (6) Kopainsky, B.; Hallermeier, J. K.; Kaiser, W. Chem. Phys. Lett. 1982, 87, 7. (7) (a) Scheibe, G. Angew. Chem. 1937, 50, 212. (b) Kandler; Ecker, H. Naturwissenschaften 1937, 25, 75. (8) Sundstro¨m, V.; Gildbro, T.; Gadonas, R. A.; Piskarskas A. J. Chem. Phys. 1988, 89, 2754.

“herringbone”, or “brickwork” structures.9-11 All the hypothesis accepted nowadays consider J-aggregates fragments where the monomers are arranged in a threadlike structure.12 The renewed interest on J-aggregates is due to the enhancement of nonlinear optical properties with the possibility of application in the field of optoelectronics.13-16 This has stimulated the study of the properties of cyanine J-aggregates, not only in solution12 but also in lowtemperature glasses17-19 and in polymer films.20,21 Since solid-state materials are needed for electronic devices, the incorporation of J-aggregates in polymeric matrixes seems to be a suitable technique. While the UV-vis absorption and fluorescence spectra of cyanine dyes have been extensively studied,3,18,19,22,23 the vibrational assign(9) Dushl, C.; Frey, W.; Knoll, W. Thin Solid Films 1988, 160, 251. (10) Czikkely, V.; Fo¨rsterling, H. D.; Ku¨hn, H. Chem. Phys. Lett. 1970, 6, 11. (11) Kirstein, S.; Steitz, R.; Garbella, R.; Mo¨hwald, H. J. Chem. Phys 1995, 103, 818. (12) Stegemeyer, H.; Sto¨ckel, F.; Bunsenges, B. Phys. Chem. 1996, 100, 9. (13) Minoshima, K.; Misawa, K.; Kobayashi, T. Chem. Phys. Lett. 1994, 218, 67. (14) Shelkovnikov, V. V.; Zhuravlev, F. A.; Orlova, N. A.; Plekhanov, A. I.; Safonov, V. P. J. Mater. Chem. 1995, 5, 1331. (15) Markov, R. V.; Plekhanov, A. I.; Rautian, S. G.; Safanov, V. P.; Orlova, N. A.; Shelkovnikov, V. V.; Volkov, V. V. Opt. Spectrosc. 1998, 85, 588. (16) Kobayashi, T. J-Aggregates; World Scientific: Singapore, 1996. (17) Scheblykin, I. G.; Drobizhev, M. A.; Varnavsky, O. P.; Auweraer, M. Van der; Vitukhnovsky, A. G. Chem. Phys. Lett. 1996, 261, 181. (18) Drobizhev, M. A.; Sapozhnikov M. N.; Varnavsky, O. P.; Auweraer, M. Van der; Vitukhnovsky, A. G. Chem. Phys. Lett. 1996, 211, 455. (19) Scheblykin, I. G.; Varnavsky, O. P.; Verbouwe W.; Backer, S. De; Auweraer, M. Van der; Vitukhnovsky, A. G. Chem. Phys. Lett. 1998, 282, 250. (20) Misawa, K.; Ono, H.; Minoshima, K.; Kobayashi, T. Appl. Phys. Lett. 1993, 63, 577. (21) Misawa, K.; Ono, H.; Minoshima, K.; Kobayashi, T. J. Lumin. 1994, 60, 812.

10.1021/la000579e CCC: $19.00 © 2000 American Chemical Society Published on Web 10/21/2000

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Chart 1. Pseudoisocyanine Iodide in the All-Trans Conformation, with the Mono-Cis Conformation Obtained by a 180° Rotation over One of the Central Chain C-C Bonds

ments of their infrared spectra have not been completely established yet.24-27,28 Recently, it has been suggested that the all-trans and the mono-cis stereoisomers of 1,1’-diethyl2,2’-cyanine may originate different types of J-aggregates.16 Moreover, the band assignments may provide a basis for discussing the influence of the aggregate’s environment on band shifts and relative intensities. Consequently, infrared and Raman techniques have been used to characterize the structure and the orientation of J-aggregates in different media, such as detergent systems29 and Langmuir-Blodgett films.30 Their orientation has also been an important issue and has been achieved in a number of matrixes, such as poly(vinyl alcohol) (PVA).20 This work aims the structural characterization, by FTIR, of cellulose acetate films dyed with 1,1’-diethyl-2,2’-cyanine iodide (pseudoisocyanine iodide, [PIC]I; Chart 1). Cellulose acetate represents an interesting material, since it is biodegradable and exhibits mineralization, a property that is crucial to an efficient disposal, namely by composting treatment.31 This method is among the emerging technologies that may contribute to a better solid waste management. Moreover, is it advantageous over other classical polymers used as matrixes, such as poly(vinyl alcohol) (PVA), which is biodegradable in solution but undergoes a very limited mineralization in solid mixtures. The films were obtained by spin coating of appropriate precursor solutions in acetonitrile.32 The spectra of the pure dye (in the crystalline state and as a film containing J-aggregates) and of the matrix (cellulose acetate, with an acetylation degree of 2.45) were obtained for comparison and the main bands assigned.24-27 In this highly substituted cellulose acetate, the acetate group may be in the C2, C3, and/or C6 positions of each pyranose ring. The most probable monomers will have a substitution degree of 2. As example, the 2,3-diacetyl derivative is shown in Chart 2. It was possible to clarify the cyanine conformation and the structure of J-aggregates, as well as their influence on the matrix structure. Experimental Section All the materials were reagent grade and have been used without further purification. Two precursor solutions have been (22) Cooper, W. Chem. Phys. Lett. 1970, 7, 73. (23) Kopainsky, B.; Hallermeier, J. K.; Kaiser, W. Chem. Phys. Lett. 1981, 83, 498. (24) Fujimoto, Y.; Katayama, N.; Ozaki, Y.; Yasui, S.; Iriyama, K. J. Mol. Struct. 1992, 274, 183 and references therein. (25) Sato, H., Kawasaki M.; Kasatani, K.; Katsumata, M. J. Raman Spectrosc. 1988, 19, 129. (26) Mejean, T.; Forel, M. T. J. Raman Spectrosc. 1977, 6, 117. (27) Yang, J. P.; Callender, R. H. J. Raman Spectrosc. 1985, 16, 319. (28) Akins, D. L.; O ¨ zcelik, S.; Zhu, H. R.; Guo, C. J. Phys. Chem. A 1997, 101, 3251. (29) Katsumata, M.; Kasatani, K.; Kawasaki, M.; Sato, H. Bull. Chem. Soc. Jpn. 1982, 55, 717. (30) Fujimoto, Y.; Katayama, N.; Ozaki, Y.; Araki, T.; Iriyama, K. Thin Solid Films 1992, 210, 597. (31) Gardner R. M., Buchanan C. M.; Komarek, R.; Dorschel, D.; Boggs C.; White, A. W. J. Appl. Polym Sci. 1994 , 52, 1477. (32) Brito de Barros, R. Cellulose acetate films dyed with pseudoisocyanine iodide: formation and orientation of J-aggregates. M.Sc. Thesis, FCUL, Lisboa, Portugal, 1998.

Ilharco and Brito de Barros Chart 2. Schematic Representation of Cellulose Acetate with an Acetylation Degree of 2 (2,3-Diacetyl Derivative)

prepared in acetonitrile (from Merck). They were 1 × 10-2 M in [PIC]I (from Sigma) and contained 5% (w/w) of cellulose acetate (obtained from Eastmann Kodak with an acetylation degree of 2.45 and average molecular weight of 30270) and 0.3% (w/w) of water (distilled and deionized). The addition of such a small amount of water favors the dissolution of the dye, therefore increasing its concentration, to stimulate the aggregation process.14 The formation of J-aggregates in this system occurs only during the spinning process and requires the addition of KI.32 This salt (from Pronolab) was added to one of the solutions in a high concentration (5 × 10-2 M). The solid-state films were obtained by spin coating with a Photo-Resist Spinner model-1 EC101 DT-R 485, Headway Research Inc. Circular glass substrates (2.5 cm of diameter) were covered with an excess amount of solution, to avoid any influence of this parameter on the final film thickness.33 In a systematic study described elsewhere,32 it has been found that a rotation speed of 2000 rpm for 120 s led to the preparation of dyed films with a thickness of 1 µm. For comparison, films of pure cellulose acetate (prepared by the same procedure from a precursor solution 5% (w/w) in polymer, with and without the addition of KI) and of [PIC]I without cellulose acetate were also prepared. All the films were stored in a desiccated atmosphere. For the infrared spectra of pure cyanine crystals, solid pellets of [PIC]I grinded with KBr (from Aldrich, FTIR grade) were prepared. All the cellulose acetate films were carefully detached from the substrate and pressed between two KBr pellets. However, the films of pure cyanine prepared without matrix were too thin to detach and therefore had to be scrapped and analyzed by DRIFT, using a Graseby/Specac Selector. All these manipulations took place under desiccated atmosphere, to avoid water vapor incorporation. The spectra were recorded with a Mattson Research Series 1 FTIR spectrometer, using a wide-band (400-4000 cm-1) mercury-cadmium-telluride (MCT) detector. In the DRIFT spectra, a narrow band (800-4000 cm-1) MCT detector was used. The sample compartment was purged with dry air. The spectra were scanned at 4 cm-1 resolution and were obtained as the ratio of 100 single-beam scans to the same number of background scans (pure KBr pellets). For the DRIFT spectra, 500 scans were necessary. The UV-vis absorption spectra of the films were scanned in a double-grating UV-vis-NIR spectrophotometer from Shimadzu (model UV-3101PC), with a 0.2 nm slit, at room temperature.

Results and Discussion The UV-vis absorption spectra shown in Figure 1 correspond to cellulose acetate dyed films obtained from different precursor solutions: (a) without addition of KI; (b) 0.05 M in KI. They were normalized at 529 nm. The spectrum a is very similar to those obtained for PIC solutions at low concentrations, where this cyanine does not aggregate.6,22,23 Its features (absorption maximum at 529 nm, a smaller band at 495 nm, and a shoulder at 460 nm) have been well characterized in solution as the vibronic progression (0 r 0), (1 r 0), and (2 r 0), respectively, of the monomer S1 r S0 transition.22 There is no significant evidence of cyanine aggregation, not even as H-aggregates, since these species would originate an (33) Meyerhofer, D. J. Appl. Phys. 1978, 49, 3993.

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Figure 1. UV-vis absorption spectra of pseudoisocyanine iodide dispersed in cellulose acetate films: a, as monomers (without addition of KI); b, predominantly as J-aggregates (with addition of KI).

intense band at around 480 nm.6,23 In spectrum b, besides the bands at 530 and 503 nm (assigned to a superposition of monomer and J-aggregate contributions20), the strong and narrow peak at 581 nm is the fingerprint of Jaggregates. The detailed structural characterization of these films was made by FTIR spectroscopy. This required a previous infrared study of pure PIC iodide (in crystalline phase and as a film containing J-aggregates, without any substrate) and of pure cellulose acetate films (obtained with and without the addition of KI). The spectra are shown in Figures 2 and 3, respectively. Given the complex structure of pseudoisocyanine, extensive coupling between vibrational modes is expected.25-27 The two regions shown in Figure 2a (12001650 and 400-850 cm-1) may give relevant information on the cyanine conformation in the crystals, particularly the strong bands at 1609, 1557, ∼1500, and 650 cm-1, related to the stretching and deformation modes of the resonant central chain.24,25 The band assignments are given in Table 1. In a first analysis, the broad band centered at ∼1500 cm-1 presents two components, at 1510 and 1500 cm-1, assigned to the C.C stretching modes, respectively in phase and out of phase, and a weak shoulder at 1481 cm-1, assigned to an in plane C.C stretching mode of the phenyl ring.34 This may be one of the modes resulting from the originally degenerate in-plane E1u modes of benzene (ν19a and ν19b, as numbered by Wilson, Decius, and Cross35). The positions and relative intensities of the central chain related bands are consistent with an all-trans conformation that has been well established for a variety of cyanine dyes in the crystalline state, by NMR and X-ray crystallography.24 In a comparison of the two spectra in Figure 2, it is apparent that, upon film deposition by spin coating, with formation of J-aggregates, the central chains of the PIC molecules keep the trans conformation and no important structural changes occur. In fact, the high steric hindrance in mono-cis conformers would modify the force constants of the central chain bonds, with the consequent shifts in the vibrational modes. The broad band centered at ∼1500 cm-1 will deserve a more detailed analysis when discussing the spectra of the dyed films. (34) Varsanyi, G. Vibrational Spectra of Benzene Derivatives; Academic Press: New York, 1969. (35) Wilson, E. B.; Decius, J. C.; Cross, P. C. Molecular Vibrations; McGraw-Hill Book Co.: New York, 1955.

Figure 2. (a) FTIR spectrum of pseudoisocyanine iodide in the crystalline state. (b) DRIFT spectrum of a film containing J-aggregates, prepared without matrix. Table 1. Band Assignments for the FTIR Spectrum of Pseudoisocyanine Iodide in the Crystalline State wavenumber/ cm-1

band assgn

intensity

ref

1609 1557 1510 1500 1481 1454 1433 1393 1381 1349 1339 1285 1246 1225 820 734 650

νCdN νC. C.N νC.C νC.C ν19 (phenyl rings) δa(CH3) δa(CH2) δs(CH3) νC.C (quinolinic rings) νC.C (quinolinic rings) νC.C (quinolinic rings) δCH2 δCH2 δCH aromatic (in plane) δCH aromatic (out of plane) δCH aromatic (out of plane) δC.C (central chain)

s s s s m w m w w m m w m m s s s

24, 27 24, 27 24, 25, 27 24, 25, 27 24 24 24 24 27 27 27 27 27 42 41, 42 41, 42 25

The band assignments of the cellulose acetate films spectra (shown in Figure 3) are made in Table 2. The main features in spectrum 3a (film obtained without addition of KI) are related to the carboxylate group and are assigned to the νCdO (at 1751 cm-1) and to the νC-O (at 1235 cm-1) modes. The strong band at 1051 cm-1 is assigned to the νC-O-C mode of the pyranose ring. Weaker, but also very informative on the intra- and intermolecular interactions, is the broad band centered at 3491 cm-1, assigned to the O-H stretching mode. Although very dependent on the medium, “free” OH groups usually absorb in the range between 3500 and 3700 cm-1. 36 Usually, a red shift to ∼3470 cm-1 is due to hydrogen bonding between an alcohol and an ether oxygen and a further shift to ∼3350 cm-1 is observed when hydrogen bonds are formed between two alcohol molecules.37

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Ilharco and Brito de Barros Chart 3. (a) Intramolecular Hydrogen Bonds between OH (at C2) and OCOCH3 (at C6′) (e.g., for a 3,6-Diacetyl Derivative) and (b) Intramolecular Hydrogen Bonds between O5 (Pyranose Ring) and HO (at C3′) (e.g., for a 2,6-Diacetyl Derivative)

Figure 3. (a) FTIR spectrum of a pure cellulose acetate film. (b) FTIR spectrum of a cellulose acetate film with addition of KI to the precursor solution. Table 2. Band Assignments for the FTIR Spectrum of Cellulose Acetate wavenumber/ cm-1 3491 2947 2890 1751 1433 1371 1235 1161 1051 904 600

band assignt

intensity

ref

νOH νCH νCH νCdO δs(CH2) or δa(CH3) δs(CH) νC-O (carboxylate) νasC-O-C (bridge) νC-O-C (pryanose ring) νas (ring) or δCH (out of plane) γOH (out of plane)

m m m s m s s w s w

36, 38, 39 38, 39 38, 39 36, 37 36, 38, 39 38, 39 36 38, 39 38, 39 38, 39

w

38, 39

Depending on the degree and positions of acetylation, two types of intramolecular hydrogen bonds may be found in cellulose acetate: an alcohol-ester bond between the OH at the C2 position and the carboxylic oxygen at the C6’ position (or vice versa); an alcohol-ether bond between the oxygen in one pyranose ring (O5) and the OH group in the C3’ position of the other (when not substituted). The former involves an oxygen atom with lower ability for electron donation, and therefore, the corresponding νO-H mode is expected to occur at a higher wavenumber. These two types of hydrogen bonds are exemplified in Chart 3a,b, respectively. Besides, the nonsubstituted OH groups at C2, C3, and /or C6 positions may be engaged in intermolecular hydrogen bonds, similar to those formed in amorphous cellulose.38 Among these, the ones involving C6 positions will occur between more distant chains, resulting in weaker hydrogen bonds. Kondo et al.39 have used regioselectively substituted methylcellulose compounds as models for examining the hydrogen bonding formation by infrared spectroscopy. In the νOH band, they have assigned the component at ∼3352 (36) Bellamy, L. J. The Infrared Spectra of Complex Molecules, 3rd ed.; Chapman and Hall: New York, 1975; Vol. 1. (37) Silverstein, R. M.; Bassler, G. C.; Morril, T. C. Spectrometric Identification of Organic Compounds, 4th ed.; Wiley: New York, 1981. (38) Ilharco, L. M.; Garcia, A. R.; Silva, J. L. da; Vieira Ferreira, L. F. Langmuir 1997, 13, 4126. (39) Kondo, T.; Sawatari, C. Polymer 1996, 37, 393.

cm-1 to intermolecular hydrogen bonds at the C2 and C3 positions and the component at ∼3447 cm-1 to those involving the C6 position. They concluded that, despite the large fwhm (full width at half-maximum) of the νOH band in most cellulose derivatives, it is narrower when intermolecular hydrogen bonds may be excluded. This situation occurs when the substituent groups are in positions C6, hindering the proximity between chains. In this case, the maximum absorption in the νOH band occurs at ∼3465 cm-1. In Figure 3a, the νO-H band is broad (the fwhm is of ∼280 cm-1) and the maximum absorption occurs at 3491 cm-1. A large concentration of KI in the precursor solution produces structural changes in the film, shown in spectrum 3b: the band is much broader (fwhm of ∼320 cm-1) indicating that there are OH groups in a larger variety of situations, but shifted to higher wavenumbers (∼3550 cm-1), suggesting that the fraction of free hydroxyl groups has increased. The deconvolution of these spectra by a nonlinear least-squares method in a sum of three Gaussian peaks yielded the results included in Table 3, lines 1 and 2. The statistical fitting parameters (χ2 and correlation coefficient) were excellent. The two important components in the spectrum of pure cellulose acetate film (without KI) are centered at 3502 and 3352 cm-1 and are assigned respectively to intramolecular hydrogen bonds of the type shown in Chart 3a and to intermolecular OH...OH bonds in positions C2 and C3. A small fraction of free OH groups must account for the smaller component centered at 3701 cm-1. In spectrum b, the predominant band is centered at 3548 cm-1 (∼78% of relative intensity), and representing ∼16% of the band, a component at even higher wavenumbers is observed. These two components may be assigned to a large majority of “free” OH groups (not involved in hydrogen bonds), part of them with the O-H stretching mode obstructed by the presence of large neighbor ions. A residual fraction of OH...OH intermolecular hydrogen bonds is also present, as indicated by the small component at 3350 cm-1 (∼6%). A very interesting conclusion is that hydrogen bonds involving the CdO groups may be excluded, since the corresponding stretching mode is at the same wavenumber in both spectra (1751 cm-1) and very close to that observed for “free” CdO ester groups (1750 cm-1).36 The pyranose ring νC-O-C mode is observed at a lower energy than the corresponding one in native cellulose (1051 versus 1060 cm-1 38), although the degree of crystallinity of cellulose acetate is comparable to this cellulose, i.e., predominantly

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Table 3. Deconvolution in Gaussian Components of the νO-H Band of Cellulose Acetate (CA) and of the νC.C Band of Pseudoisocyanine Iodide, for Pure and Dyed Films with a Nonlinear Least-Squares Method Used sample

band

peak center

fwhm

rel area

χ2

F

pure CA film

νOH

3701 3502 3352

265 163 326

14.19 41.46 44.34

4.869 × 10-7

0.9993

CA film with KI

νOH

3794 3548 3350

203 249 159

15.87 77.64 6.49

5.132 × 10-7

0.9972

CA film with [PIC]I monomer

νOH

3564 3494 3391

65 107 132

10.19 68.97 20.85

3.014 × 10-7

0.9995

νC.C

1529 1513 1502 1492

12 17 7 16

6.33 61.18 5.29 27.20

2.1858 × 10-7

0.9997

νOH

3543 3448 3471 3352

150 150 50 206

28.82 26.80 1.53 42.85

1.1724 × 10-7

0.9993

νC.C

1511 1502 1494 1484

17 7 12 11

55.32 7.41 22.72 14.55

2.8297 × 10-7

0.9967

νC.C

1516 1510 1498 1479

17 6 20 12

35.19 3.84 49.08 11.89

7.7822 × 10-6

0.9995

CA film with [PIC]I J-aggregates

[PICI]I cryst state

Figure 4. FTIR spectra of cellulose acetate films dyed with pseudoisocyanine in the monomer form (a) and as J-aggregates (c). The spectra of the corresponding matrix films are included for comparison in lines b and d, respectively.

amorphous. This result can be assigned to a lower degree of intramolecular hydrogen bonding involving this oxygen in cellulose acetate. In fact, that stretching mode involves the vibration of the entire ring and is expected to be more hindered in cellulose, where there are pronounced hydrogen bonds between O5 and OH at C3’. In conclusion, the presence of KI in pure cellulose acetate films has two important effects: it induces the separation of the polymer chains (by breaking intermolecular hydrogen bonds) and increases the stretchability of this matrix (by also breaking intramolecular hydrogen bonds). The infrared spectra of the dyed films are shown in Figures 4 and 5 in the interesting regions of the matrix and of the cyanine, respectively. In Figure 4, the same spectral regions of pure cellulose acetate films prepared with and without addition of KI are included for comparison. The maxima of the νOH, νC)O, and νC-O-C (pyranose

ring) modes of cellulose acetate in the monomer-containing films (Figure 4, line a) respectively at 3489, 1751, and 1051 cm-1 are very close to those observed in the pure cellulose acetate films (line b) (3491, 1751, and 1051 cm-1). However, the OH band is much narrower (fwhm ∼150 cm-1), indicating that the OH groups are engaged in a predominant type of hydrogen bonds. This is confirmed by the deconvolution of the band, performed in a similar way to that described above and summarized in Table 3, line 3. It reveals a largely major component at 3494 cm-1 (∼69% of relative intensity). The fraction of free OH groups decreased (only ∼10% of the band), and a nonnegligible component appears at 3391 cm-1 (∼21%). Therefore, the presence of the dye in the monomeric form induces the breaking of some intermolecular hydrogen bonds, allowing the participation of the corresponding OH groups in intramolecular hydrogen bonds with the carboxylate oxygens (as in Chart 3a). The influence of the J-aggregates on the arrangement of the polymeric chains is apparent from the modifications between spectra 4c and 4d. In fact, the large red shift of the νOH mode (∼100 cm-1) when the film contains J-aggregates proves that the OH groups of the matrix participate in more extensive hydrogen bonding. The decrease in the fwhm (from ∼320 to 230 cm-1) is an indication that the presence of J-aggregates limits the diversity of the interactions in which the OH groups are involved, perhaps favoring a specific type of hydrogen bond. The deconvolution of the OH band in spectrum 4c was only satisfactory with four Gaussians (Table 3, line 5). There are two important components, at 3352 (∼43% of the band intensity) and 3448 cm-1 (∼27%). The first is characteristic of intermolecular hydrogen bonds between OH groups in the C2 and C3 positions and the second between OH in the C6 positions, as referred above. These do not involve the carbonyl groups, since the red shift of the νCdO mode is negligible (∼5 cm-1) and the bandwidth change lies within the spectral resolution. Although in the films containing J-aggregates the intramolecular interactions described in Chart 3b are not excluded, the

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Ilharco and Brito de Barros

Chart 4. Intermolecular Hydrogen Bonds between OH at C2, C3, and/or C6 Positions (Predominant in Films with J-Aggregates)

component at 3470 cm-1 represents only 1.5% of the band intensity, indicating that this type of hydrogen bond is not very significant in this system. The fraction of free OH groups, which represents 94% in the cellulose acetate film containing the same amount of KI, is dramatically decreased to 29%. A pictorial example of the predominant intermolecular hydrogen bonds between cellulose acetate chains is given in Chart 4. The enhancement of intermolecular hydrogen bonding between the C2 and C3’ positions reveals a greater proximity between the cellulose acetate chains, due to the presence of cyanine J-aggregates. This does not correspond to an increased crystallinity, since polarized microscopy revealed that these films are almost amorphous.32 Previous findings on the effect of model disperse dyes in cellulose acetate films had correlated the decrease in the diffusion coefficient of the dye upon aggregation with an increased compactness of the amorphous phase rather than to a higher film crystallinity.40 Accordingly, we attribute the changes in the OH stretching band of cellulose acetate to a higher degree of packing of the amorphous phase, induced by the presence of cyanine J-aggregates. These modifications in the νOH band of the matrix may provide information on the aggregation state of the dye. In the spectra of the dyed films, the matrix bands are bound to dominate. However, the cellulose acetate shows convenient spectral “windows” in the most interesting regions of the dye (1450-1650 and 620-850 cm-1), thus enabling the analysis of cyanine bands. Remarkable changes in the infrared bands of the central chain would be expected if the dispersion of the dye in the matrix were accompanied by important modifications in the monomer conformation. In a comparison of the spectra in Figures 2 and 5 (in the cyanine wavenumber windows), it is clear that the bands assigned to the νC.N and νC.C (.N) modes are very similar in the crystals and in films containing monomers and J-aggregates, as the shifts are within the spectral resolution. Thus, we may conclude that the pseudoisocyanine iodide molecule assumes a similar conformation (the all-trans shown in Chart 1) in the crystalline state, dispersed as monomers and as J-aggregates in cellulose acetate. Four cyanine bands deserve further attention: the broad band centered at ∼1500 cm-1 (assigned to νC.C modes), the weak one at ∼820 cm-1 (assigned to the out of plane δC-H in the cyanine (40) Papadokostaki, K. G.; Petropoulos, J. H. J. Polym. Sci., Part B: Polym. Phys. 1994, 32, 2347.

Figure 5. FTIR spectra of pseudoisocyanine iodide dispersed in cellulose acetate films as monomers (a) and as J-aggregates (b).

heterocyclic moieties), and the one at ∼730 cm-1 (assigned to the same out of plane deformation of the phenyl rings).41 The broad band around 1500 cm-1 shows at least three components, whose relative intensities are apparently different, whether the cyanine is crystalline, dispersed in films containing just monomers, or predominantly Jaggregates. Interestingly, the structure of this band in films containing J-aggregates does not seem to depend on the presence of a matrix (it is the same in spectra 2b and 5b). By deconvolution, this band is indeed composed and the best fit was obtained with four Gaussians. The results are shown in Figure 6 and Table 3, lines 4, 6, and 7. The central-chain-related components are always very intense, at 1513 cm-1 (61% of total intensity for dispersed monomers) and 1511 cm-1 (55% for dispersed J-aggregates) and the doublet 1510/1516 cm-1 (∼39% in crystals). Related to the aromatic νC.C modes in all the spectra are the bands at 1492/1502 cm-1 for dispersed monomers, 1494/1502 cm-1 for J-aggregates, and 1498 cm-1 for crystals. This band is more intense in the crystalline phase (∼49% of relative intensity versus 30% and 32% respectively for films with J-aggregates and just monomers), possibly due to a solid-state resonant effect. (41) Karr, C.; Estep, J. P. A.; Papa, A. J. J. Am. Chem. Soc. 1958, 81, 152. (42) Rao, C. N. Chemical Applications of Infrared Spectroscopy; Academic Press: London and New York, 1968.

Pseudocyanine Iodide in Cellulose Acetate Films

Langmuir, Vol. 16, No. 24, 2000 9337

Figure 6. 6. Deconvolution in four Gaussians of the νC.C band of the FTIR spectra of pseudoisocyanine iodide in the crystalline state (A), as monomers dispersed in a cellulose acetate film (B), and as J-aggregates dispersed in a cellulose acetate film (C). Chart 5. Schematic Representation of the Arrangement of Pseudoisocyanine Monomers in a J-Aggregate Dispersed in Cellulose Acetate Films

More significant is the low-frequency component at ∼1480 cm-1, which appears with similar relative intensities in the crystals and J-aggregate spectra but which is absent in the monomer spectrum. It is also assigned to a C.C stretching mode of the aromatic rings, affected by the proximity and, probably, the high overlap of the phenyl rings. This result clearly shows that in J-aggregates there is a superstructure resembling that of the crystalline phase. Upon aggregation, the band at 818 cm-1 (related to the out of plane C-H deformation of the heterocyclic ring) does not change in frequency or relative intensity (taking as reference the constant band at 1557 cm-1). On the contrary, the one at ∼730 cm-1 is relatively weaker and broader in the monomer containing films (fwhm ) 22 cm-1, versus 15 and 10 cm-1 for spectra 5b and 2a, respectively). This result reinforces the similarities between J-aggregates and a crystalline state of cyanine. Moreover, and more important, it confirms the previous conclusion that the formation of J-aggregates involves an overlap of the quinolinic rings but just or mainly of the phenyl rings, without conformational changes. This is a good evidence for a threadlike structure of the J-aggregates, as shown in Chart 5.

responsible for a higher degree of packing of the cellulose acetate amorphous phase, partially breaking intramolecular hydrogen bonds and inducing the formation of a larger number and variety of interchain hydrogen bonds. This effect is particularly strong, since these films contain a high concentration of an ionic salt (KI, necessary for the dye aggregation), which induces the breaking of intraand intermolecular hydrogen bonds in cellulose acetate, leaving ∼94% free hydroxyl groups. Complementary information on the structural arrangement of the cyanine molecules in J-aggregates was obtained. Namely, from the vibrational modes of the central chain, it was possible to conclude that the pseudoisocyanine iodide molecules acquire the all-trans conformation, both in the monomeric and aggregated forms. The arrangement of the cyanine molecules in J-aggregates approaches that of the crystalline phase. The modifications in the out of plane C-H deformations are consistent with an overlap of the phenyl rings of the monomers to form a J-aggregate, without affecting the heterocyclic rings. Additionally, it was shown that the cellulose acetate matrix does not play an important role on the arrangement of the monomers in J-aggregates, as the same structure is obtained for films produced without a polymeric matrix.

Conclusions The FTIR characterization of cellulose acetate films (pure and dyed with PIC iodide) allowed one to understand the structural perturbations induced on the matrix by the dye when it forms J-aggregates. These species are

Acknowledgment. R.B.B. acknowledges the Fundac¸ a˜o para a Cieˆncia e a Tecnologia (FCT) for the grant PRAXIS XXI BM1280. LA000579E