The Phthalocyanine–Polystyrene Composite: Tuning the Optical

ARTICLE pubs.acs.org/JPCC

The Phthalocyanine
Polystyrene Composite: Tuning the Optical Spectrum via Polymer-Triggered Phase Transformations of the Dye Nicholas Yu. Borovkov* and Svetlana V. Blokhina Institute of Solution Chemistry, Russian Academy of Sciences, 1 Akademicheskaya Street, 153045, Ivanovo, Russia

bS Supporting Information ABSTRACT: This work reports on copper tetra(3-nitro-5-tertbutyl)phthalocyanine (CuPc*) exhibiting complex but wellcontrolled phase behavior. A key structural feature of this dye is the thermodynamically stable χ-type phase whose optical absorbance is red-shifted with respect to the CuPc* molecule. Small-angle X-ray scattering and optical spectroscopy have been used to watch the phase formation of CuPc* in the onecomponent system and binary composite with polystyrene (PS). The polymer exerts a powerful stabilizing effect on the lower phases of CuPc*, hence a manageable suite of the phase transformations βfRfχ is realized in the CuPc*
PS system. Regardless the phase type, the composite is highly transmissive in the blue-green region of the spectrum. Thus, it is applicable to reveal quantitative relationships between nonlinear optical properties and the supramolecular structure of phthalocyanines.

1. INTRODUCTION A keen interest is being evoked in organic functional materials whose physical properties may be tuned in a controlled manner.1 In particular, phthalocyanines, which are organic dyes with extraordinary photophysical properties, photochemical stability, and complex aggregation behavior, are considered a versatile chemical platform for the development of such materials.2
5 Of major interest are soluble phthalocyanines, in particular tert-butylsubstituted ones, studied as optical materials in one-component films,6,7 suspended state,8 and dye
polymer composites.9
11 Generally, aggregation of phthalocyanines is thought to be undesirable since strong intermolecular interactions reduce functional properties including the nonlinear absorption. However, in the case of copper tetra-tert-butyl-phthalocyanine (CuPc0 , Figure 1), this phenomenon seems applicable for tuning nonlinear optical behavior.8 Researchers8 dealt with the thermodynamically stable R-type phase12 constructed from CuPc0 aggregates with extensive overlapping of adjacent macrocycles.13 Considering a variety of the phthalocyanine aggregated species1,14 from the traditional viewpoint,3 one may note that researchers should pay particular attention to the χ-type phase where the said overlapping is minimal because of the supposedly staggered intrastack molecular arrangement.15
19 Although the opportunity to construct the staggered stack of the CuPc0 molecules was shown more than 20 years ago,20 the hardly controlled phase behavior of the dye hampered integrating this finding into the material science field. As one may judge from the optical spectra of different tetra-tertbutyl-phthalocyanines reported so far, the stable χ-type phase has been obtained only for the antimony derivative.21 In accordance with modern concepts of the phthalocyanine material design,3,5,14 various functional groups are appended to r 2011 American Chemical Society

the phthalocyanine molecule to modify the aggregation behavior thereof. In particular, a fruitful approach22
24 recommends appending the oxyethylene fragments terminated with one or another group of the aromatic nature. In such a way, highly ordered arrays of the rod-like phthalocyanine aggregates25,26 with novel electrophysical properties27 have been constructed. As for the optical material design, this approach is hardly applicable for two reasons. First, as the rod-like aggregates0 spectra28 indicate, the peripheral substitution at the 4- and 5-atoms of the benzene rings stabilizes only the R-type phase as stated above. Second, strong interstack interactions between the phthalocyanine “rods” greatly enhance the near-UV absorbance29 and, thus, make “cloudy” the optical window between 450 and 550 nm where the triplet
triplet transition bands are located.30 Recently researchers31 have demonstrated that lateral substitution at the 3- and 6-atoms of the benzene rings stabilizes the χ-type phase whose optical absorbance15,16 is red-shifted with respect to the molecular form. Therefore it is likely that appending a lateral fragment to the benzene rings of the CuPc0 molecule will result in the intermediate case, namely, comparable thermodynamic stabilities of the different phases and hence opportunity to change the optical spectrum of the dye via controlled phase transformations. Ten years ago, the authors dealt with the aggregation behavior of copper tetra(3-nitro-5-tert-butyl)phthalocyanine (CuPc*, Figure 1).32 This dye is of interest as a component of optical materials because of its well-balanced molecular structure: the Received: March 10, 2011 Revised: August 1, 2011 Published: August 05, 2011 17945

dx.doi.org/10.1021/jp206072m | J. Phys. Chem. C 2011, 115, 17945–17951

The Journal of Physical Chemistry C

ARTICLE

Figure 1. Phthalocyanines under investigation (depicted as the C4h isomer).

tert-butyl-groups ensure compatibility with organic media, whereas the nitro-groups serve as a molecular modulator of the nonlinear optical properties.6 The CuPc* aggregates on the water surface were preliminarily interpreted as low-dimensional prototypes of the crystalline β-, R-, and χ-phases. An optical spectrum of the CuPc* film (not given in the work32) shows a key attribute of all χ-phase phthalocyanines,15,16,33
35 namely, an intense and symmetrical band mimicking the Q-band of the electronically isolated molecule (cf. curves 1 and 2 in Figure 2) but red-shifted. Therefore the present work has a twofold objective: first, to clarify the main features of CuPc* aggregation in the solid state; second, to estimate physicochemical opportunities for preparing the CuPc*-based optical materials.

2. EXPERIMENTAL SECTION 2.1. Materials. Spectrograde solvents and polystyrene (PS) with a weight average molecular weight of 390 000 were acquired from Aldrich (USA). Phthalocyanine CuPc* synthesized and finely purified by conventional procedures32 consisted of two nonisomorphous fractions. To separate the fractions at the preparative level, a CuPc* sample was dissolved in pyridine, deposited onto silica (Chemapol, Czech Republic) and placed onto a top of a chromatographic column. The column was flushed with methanol to remove excessive pyridine. Then two fractions (denoted CuPc*A and CuPc*B) were eluted successively with acetone and benzene. The CuPc*A/CuPc*B ratio was ca. 1:10. The fractions were characterized by the1H NMR spectroscopy and X-ray scattering techniques, the data being placed in the Supporting Information. 2.2. Techniques. The Langmuir
Blodgett (LB) films were prepared on a NIMA Langmuir trough. Preparation of the floating layers was described earlier.32 The compressed monolayers were transferred onto hydrophobic silicon plates by vertical lift at transfer pressures of 4 and 20 mN/m. The cast films were prepared as usual:9
11 the dye/polymer solutions in a pyridine/benzene mixture (ca. 1:50, vol.) were drop-cast into Petri-dish-like cuvettes and allowed to desolvate at room temperature; residual pyridine was removed by vacuum heat treatment at 60 °C. X-ray scattering measurements on the LB films were performed on a Rigaku Denki RV300 diffractometer equipped with a rotating anode. The Ni-filtered Cu
KR radiation was used. Optical spectra were recorded on a Specord M400 spectrophotometer controlled by IBM PC. The LB film morphology was observed by a variety of microscopy techniques, the data being placed in the Supporting Information.

Figure 2. Optical spectra of the phthalocyanines (Figure 1): curve 1, CuPc*B (benzene solution); curve 2, CuPc*B (cast film); curve 3, CuPc0 (cast film); curve 4, CuPc*A (benzene solution); curve 5, CuPc*A (cast film); curve 6, CuPc*A (cast film annealed 1 h at 120 °C). The spectra are normalized to the maximal optical density. The A and B superscript indexes designate two fractions of CuPc* (see the Experimental Section).

3. RESULTS AND DISCUSSION 3.1. General Consideration. The addition of four nitrogroups to the CuPc0 molecule weakly affects the UV
vis spectrum in diluted solutions but drastically changes the aggregation behavior. Whereas CuPc0 is absolutely compatible with low-polar solvents, such as benzene, CuPc* is well soluble only in pyridine. Thus, the π
π coupling that mainly drives phthalocyanine aggregation is significantly more powerful in the latter case. All the same, mixing a concentrated CuPc* solution in pyridine with a poorer solvent allows preparing the homogeneous multicomponent systems, including the dye
polymer ones. Such a feature indicates the formation of a molecular CuPc*
pyridine complex being stable in organic media at room temperature. This phenomenon helped us to solve important preparative tasks: partial separation of the CuPc* isomers and casting the vitreous CuPc*
PS films. Any phthalocyanine synthesized from an asymmetrically substituted phthalic acid is known to be a mixture of four isomers with different aggregation behaviors36 but tending to random coaggregation.37 The CuPc* formula in Figure 1 depicts the C4h isomer with the isometric molecular shape. Usually this isomer constitutes a minor part: to be exact, 12.5% of the whole sample. The major part consists of the asymmetric C2v and Cs isomers (75.0%); the rest is the anisometric D2h one. Thus, theoretically the CuPc*A fraction may consist of either C4h or D2h isomer. Although the isomers can not be identified by 1H NMR spectroscopy, a reasonable assumption about the composition of two fractions has been made on the basis of the optical behavior explicated below. 17946

dx.doi.org/10.1021/jp206072m |J. Phys. Chem. C 2011, 115, 17945–17951

The Journal of Physical Chemistry C

Figure 3. Hypothetical molecular arrangements of the CuPc* aggregates analogous with the β (left) and χ (right) phases of unsubstituted phthalocyanines. The structures are sketched for the D2h and C4h isomers of CuPc*, respectively.

The fractions are optically identical in solution (Figure 2, curves 1 and 4); the Q-band is blue-shifted with respect to one in the CuPc0 spectrum only by 3 nm. However, in the solid state, the spectra of three specimens under consideration differ drastically. In the case of reference CuPc0 , a shouldered one-band profile in the visible region (curve 3) indicates the R-type phase.9 The CuPc*B spectrum profile (curve 2) indicates the χ-type phase whose spectrum exhibits even a kind of vibration satellite at ca. 640 nm. The third profile (curve 5) has two partially resolved bands and hence indicates the β-type phase9 to be stable for CuPc*A at room temperature. Upon heating, the CuPc*A spectrum becomes one-banded as a result of the temperature-induced transformation βfχ (cf. curves 5 and 6). Two fractions yield drastically different solid particles (see the optical micrographs in the Supporting Information). CuPc*A is deposited from pyridine as transparent prismatic microcrystals whose habit and size are independent from the desolvation conditions. In contrast, vitreous films of CuPc*B are easily prepared by drop-casting provided the solvent is eliminated rapidly. Under slow desolvation conditions, corrugated plate-like CuPc*B particles arise. Such behavior, being a general symptom of amorphousness of phthalocyanine films,38 seems to be connected with the geometrical nonhomogeneity of the CuPc*B fraction. Here one should also note that the pyridine-complex formation is a prerequisite of successful preparative separation of the CuPc*B fraction on the quinoline-grafted silica.39 Although the said features of CuPc* are revealed by the quite superficial experiment, they allow composition of an important structural scheme sketched in Figure 3. The scheme implies that phenomena such as the stable χ-type phase and the inverted transformation βfχ are determined by the molecular shape factor being a cooperative repulsion between all tert-butyl- and nitro-groups of the stacked molecules. In particular, the staggered stack may be stable when constructed from the conveniently accommodated isometric molecules (the right structure), whereas the strongly shifted stack (the left structure) may be favored by the cross-wise accommodation of the anisometric molecules. Thus, most probably the CuPc*A fraction consists of the D2h isomer. Two asymmetric isomers seem isomorphous with the C4h one, as evidenced by a lack of dependence of the CuPc*B film spectrum on a solvent used for casting. The aggregation behavior of the two CuPc* fractions allows one to classify the phases by thermodynamic stability. In the case of CuPc*A, the difference in stability between the β- and χ-type phases is obviously lower than the activation barrier of the transformation βfχ. Therefore, both may be considered as stable.

ARTICLE

Figure 4. X-ray diffraction patterns of the thick LB films prepared by 100-fold dipping into the CuPc*B layers at 20 mN/m. Curves 1 and 2 refer to the films prepared from the well-compressed monolayer and nonorganized free-floating multilayer, respectively. The decimals show the interlayer periodicities (nm). Dimensions of the CuPc* molecule (the C4h isomer) are given in the inset.

In the case of CuPc*B, only the χ-type phase is stable because none of its precursors manifest themselves in the cast film. 3.2. The LB Films. To provide insight into structural prehistory of the stable χ-type phase, the aggregation behavior of CuPc*B has been studied in a following way: the floating layers have been prepared under various conditions,32 transferred onto a solid support, and observed by the X-ray scattering technique for several days until the stable structural state was achieved. A typical diffraction pattern (Figure 4, curve 1) has one diffuse peak corresponding to the interlayer periodicity of 2.3 nm. This value falls between 1.8 and 2.5 nm (the inset) and, thus, is close to the maximal molecular dimension. The correlation length determined from the Scherrer equation is ca. 8.7 nm. It indicates that the upright ordering extends over four one-stack aggregates but not more. On this account, one may adopt the LB film to consist of the poorly ordered one-stack aggregates with the staggered molecular arrangement. The metastable phase states become observable when the CuPc*B films are prepared from the free-floating or slightly compressed layers. In particular, a curious structure has been observed in the film prepared from the noncompressed layer deposited by spreading a very large quantity of the dye solution, i.e., under conditions when the desired transfer pressure is achieved without compression and the aggregates are allowed to form spontaneously. In such a case, the diffraction pattern (Figure 4, curve 2) has several adjacent peaks corresponding to the average interlayer periodicity of 2.6 ( 0.1 nm. Other complex structures have been observed in the films prepared from the slightly compressed monolayers. Figure 5 traces the structural evolution in one such film. Initially the pattern looks like a wide spectrum of periodicities with two regions of relatively marked intensity. The high-periodicity structures (d = 4.6 ( 0.6 nm) dissociate in several days, and finally the familiar χ-type phase structure (d = 2.4 ( 0.1 nm) appears. The X-ray scattering data presented indicate coexistence of at least two phases, the metastable one being constructed from the multistack aggregates. Hypothetically this phase may be classified as either a β or R one. To come to a substantiated decision, one should take into account the correlation length above. In such a case, the characteristic size of the hypothetical one-stack aggregate of the metastable phase should be adopted as ca. 1.2 nm. On the other hand, in the case of the β-type aggregate where the 17947

dx.doi.org/10.1021/jp206072m |J. Phys. Chem. C 2011, 115, 17945–17951

The Journal of Physical Chemistry C

ARTICLE

Figure 6. Normalized optical spectra of the CuPc0
PS composite. Dye content is the number of phthalocyanine moles per 1 kg of PS.

Figure 5. Successive traces of the X-ray diffraction pattern of the thin LB film prepared by 25-fold dipping into the CuPc*B monolayer at 4 mN/m. The film was allowed to age naturally at room temperature. The decimals are as in Figure 4.

angle between the molecular plane and main axis is close to 50°, the characteristic size may be estimated as ca. 1.4 nm. The corresponding value for the R-type aggregate is close to 1.6 nm. Thus, most probably the χ-type phase of CuPc*B is preceded by the β-type one being constructed from both two- and four-stack aggregates. The surface morphology of the LB films is amply presented in the Supporting Information. Generally it is close to one of the cast CuPc*B films prepared by rapid desolvation; namely, only flaws rather than intrinsic structural features are well observable. However, some films exhibit sporadic spots of intense structure formation. The observed microstructures are plate-like and, thus, morphologically analogous with the corrugated plates said above. Additionally, unusual mesoscopic vein-like formations have been revealed. Outwardly they are similar to the wire-like aggregates of the unsubstituted analogue40,41 and preliminarily may be rationalized as a subphase enriched with the asymmetrical isomers of CuPc*. In conclusion, one should note that the LB films of reference CuPc0 do not exhibit the high-periodicity structures in X-ray diffraction patterns. However, the multistack aggregates of R-phase CuPc0 were registered by the transmission electron diffraction technique.12 Elsewhere42 the aggregates were shown to assemble spontaneously into mesoscale domains. This difference between the CuPc0 and CuPc* films may be rationalized as follows. In the former case, the interstack interactions within the three-dimensional (3D) R-type aggregate are of van der Waals nature but strong enough to promote mesoscale structure formation. In the latter case, the interactions within the 3D β-type aggregate occur by the powerful π
π coupling mechanism. As a result, the multistack supramolecular entities with the collective π-electron levels43 arise. Upon the phase transformation βfχ, the interstack charge transfer becomes physically impossible, and the entities dissociate. However, the domains do not arise, because low mobility of the CuPc* aggregates in the solid state hampers the mesoscale self-assembling. 3.3. The Phthalocyanine
Polymer Composites. The handiest tool for observing the dye aggregation is known to be the optical spectroscopy technique. Conventionally, it is applied to liquid systems composed of a soluble phthalocyanine and organic

Figure 7. Normalized optical spectra of the CuPc*B
PS composite. Dye content is as in Figure 6.

solvent.44,45 A more rational approach9 recommends the use of a polymeric solvent with the low glass-to-rubber transition temperature, such as PS. A dye
polymer system has a number of advantages over a liquid one, the key ones being a full range of the available dye/solvent ratios and opportunity to compare dyes with different solution behaviors. Figure 6 presents an optical pattern resulted from concentration-induced aggregation of reference CuPc0 . The dye suffers the transformation βfR being inversed with respect to the classic transformation Rfβ for CuPc.46 The state of CuPc0 in PS may be conveniently described by limiting concentrations of phase existence: the β-type phase sustains increasing the dye content up to ca. 5  10
3 mol/kg, whereas the pure R-type phase exists at the dye content more than 1.2 mol/kg. An optical pattern resulted from concentration-induced aggregation of CuPc*B (Figure 7) indicates that CuPc*B exists in the molecular form below 1  10
3 mol/kg. Thus, despite the strong differences in solution behavior of these dyes, the aggregation thresholds are practically equal. Upon increasing the dye content, the β-type phase manifests itself in a peculiar manner: it practically covers the whole range of the dye/polymer ratio and does not show any symptoms of instability despite an increase in the aggregation number evidenced by slow but sure spectrum broadening (cf. curves 3
5). The most dye-rich sample contains only ca. 8% mass PS. Thus, the polymer exerts a powerful stabilizing effect on the metastable β-type phase of CuPc*B and may be appropriately classified as a molecular trigger.47 17948

dx.doi.org/10.1021/jp206072m |J. Phys. Chem. C 2011, 115, 17945–17951

The Journal of Physical Chemistry C

Figure 8. Non-normalized optical spectra of the CuPc*B
PS composite suffering the complex phase transformation βfRfχ (the curves 1, 2, and 3, respectively). Dye content is 0.175 mol/kg.

Switching off the transformation βfχ of CuPc*B is analogous with stabilizing the β-type phase of CuPc*A by the molecular shape factor. Hence the PS additive seems to increase the activation barrier of the transformation by purely physical impact on the multistack β-type aggregates. At the same time, there is a curious difference between the CuPc*A and CuPc*B
PS cases. Annealing the composite results in the optical spectra (Figure 8) indicating the formation of the R-type phase as an intermediate state. Thus, the full suite of the phase transformations is as follows: βfRfχ. Obviously a rational interpretation of the trigger effect should explain three features of PS: switching off the transformation βfχ, indifference to the transformation βfR, and stabilizing the intermediate R-type phase. The trigger mechanism may be rationalized if the principal structural differences between the β- and χ-type aggregates (see section 2.2) are taken into account. In general, the transformation βfχ should occur as follows: Upon increasing the dye content, the small 4-stack β-type aggregates merge in the face-to-face mode, and the intrastack π
π coupling, for so far unknown reasons, takes another physical mechanism. As a result, the shifted molecular arrangement becomes less favorable than the staggered one, and the molecules try to pivot on their main axes. For such a maneuver, a free volume is needed. The volume is deficient in the four-stack aggregate whose inner stacks are jammed. Therefore changing the intrastack arrangement should be preceded by separation of the individual stacks. Seemingly, the PS macromolecules fill up the interaggregate free space, and separation proceeds as slowly as a flow of any small particles in a viscous medium. In other words, the polymer acts as a temperature-controlled rheological gate and, thus, conserves the multistack structure. In the case of the transformation βfR, no stack separation is needed because of the shifted intrastack arrangement in both phases.1 As a result, the transformation βfR does not seem to be sensitive to the free volume shortage. For the same reason, temperature-induced dissociation of the β-type aggregate in the PS solvent initially yields the R-type one. 3.4. Spectrum Profile of the χ-Type Phase. The χ-phase of unsubstituted phthalocyanines has a narrow optical window in the blue-green region of the spectrum and therefore was studied as a nonlinear optical material only in the IR region.48 By contrast, the χ-type phase of CuPc* is even more transmissive than the R one because of a wider gap between the UV and visible bands (cf. curves 2 and 3 in Figure 2). Other features of the χ-phase spectrum are the red band located at 715 nm instead of

ARTICLE

Figure 9. Normalized optical spectra of the χ-phase: curve 1, C4h-CuPaz0 (LB film from benzene); curve 2, C4h-CuPaz0 (LB film from hexane); curve 3, CuPc (cast film from trifluoroacetic acid); curve 4, CuPc* (as in Figure 2).

the expected 780 nm and a lack of the “vestigial” bands resulting from the π
π* transitions in the electronically isolated molecule and the lower (supposedly bimolecular16) aggregate. So far, optical features of the χ-phase have only been interpreted for unsubstituted phthalocyanines. Although three different versions of the π
π coupling inherent in the χ-phase were proposed,13,16,35 none seem applicable to explain the optical differences between CuPc* and CuPc. On this account, the fourth version would not be out of place. To begin with, the optical spectra of tetra-tert-butyl-porphyrazine (CuPaz0 ) presented in our earlier work49 should be taken into account. Its isometric isomer (C4h-CuPaz0 ) has a swastikalike molecular geometry that allows, despite severe sterical hindrances to the π
π coupling, both the regular intrastack staggering and efficient interstack interactions. The spectra of the porphyrazine χ-phase (Figure 9, curves 1 and 2) exhibit the IR band red-shifted with respect to the Q-band by 255 nm, which is 2.5 times as high as the shift in the CuPc spectrum! A unique feature of C4h-CuPaz0 is the variability of the spectrum profile: changing the solvent nature allows the formation of the LB films with either well-resolved (curve 1) or nearly quenched (curve 2) “vestigial” bands. Accordingly the profile may resemble one of the χ-phase of either CuPc (cf. curves 1 and 3) or CuPc* (cf. curves 2 and 4). Also, the interplay between the IR, UV, and “vestigial” bands is clearly observable. Further, two electronic features of phthalocyanine aggregates should be recalled: first, the π
π coupling between the molecular units within the aggregate results in the formation of collective π-electron levels;43 second, the configuration mixing of these levels results in the complex nature of some bands.50 Now comparing the CuPc* and C4h-CuPaz0 spectra allows one to note that the red/IR band has its origin in the electronic transitions from the collective levels derived from both the highest and second occupied molecular orbitals (HOMO and SOMO, respectively). In the case of C4h-CuPaz0 , the intensity of the IR band is primarily determined by the SOMO-derived levels as the highly resolved “vestigial” bands at 585 and 620 nm indicate. The CuPc* dye seems to be the contrary case: the HOMO-derived levels are fully involved into the red-band transition, whereas a contribution of the SOMO-derived ones is small as the similarity between profiles of the UV bands in the β- and χ-phases indicates. As a result, the probability of the “vestigial” intramolecular π
π* transitions in the χ-type aggregate is close to zero. Accordingly, the case of CuPc seems to be intermediate. 17949

dx.doi.org/10.1021/jp206072m |J. Phys. Chem. C 2011, 115, 17945–17951

The Journal of Physical Chemistry C The statement above suggests that the χ-type phase is not a unique physical entity but a family of subphases having a common specific feature of the red/IR band. In our opinion, a variety of 3D species (the τ-, δ-, ε-phases,1 etc.) with practically identical X-ray diffraction patterns in the small angle region is constructed from the structurally similar but differently packed χ-type aggregates and hence should be considered as “daughters”17 of the parent χ-phase. For a better insight into the genesis of the χ-phase, an opticostructural study on C4h-CuPaz0 is in progress. In conclusion, one should note that CuPc* is a phthalocyanine whose complex phase transformations are not only observable but fully controlled. The authors believe that it deserves close attention as a model object to reveal the quantitative structure
property relationship for phthalocyanine-based optical materials.

4. CONCLUSIONS CuPc* exhibits a thermodynamically stable χ-type phase with complex structural prehistory revealed by small-angle X-ray scattering and optical spectroscopy. PS stabilizes the lower phases of the dye; therefore a suite of the phase transformations βfRfχ is observable in the binary CuPc*
PS system. The χ-type phase of CuPc* has an odd optical feature: the IR band is strongly blue-shifted with respect to the one in spectra of unsubstituted phthalocyanines. The feature is rationalized by the complex nature of the π
π coupling in the χ-type aggregate. Regardless of the phase type, the composite is highly transmissive at wavelengths from 450 to 550 nm and seems potentially usable as a nonlinear optical material. ’ ASSOCIATED CONTENT

bS

1

H NMR spectra of CuPc* (one isomer) and CuPc (four isomers); X-ray powder diffraction pattern of CuPc* (three isomers); optical microscopy images of the cast samples of CuPc* (one and three isomers; rapidly and slowly desolvated); SEM and AFM images of the LB films of CuPc* (three isomers; diverse preparation conditions). This information is available free of charge via the Internet at http://pubs.acs.org. Supporting Information. 0

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Phone: 7(4932)351545. Fax: 7(4932)336246.

’ ACKNOWLEDGMENT The work is partially supported by the Russian Academy of Sciences (Contract 03-18). The authors gratefully acknowledge an experimental contribution by Doctor L. A. Valkova. The content of this work is the sole responsibility of the authors. ’ REFERENCES (1) Law, K.-Y. Organic photoconductive materials: Recent trends and developments. Chem. Rev. 1993, 93 (1), 449–406. (2) Inabe, T.; Tajima, H. Phthalocyanines
Versatile components of molecular conductors. Chem. Rev. 2004, 104 (11), 5503–5533. (3) de la Torre, G.; Vazquez, P.; Agullo-Lopez, F.; Torres, T. Role of structural factors in the nonlinear optical properties of phthalocyanines and related compounds. Chem. Rev. 2004, 104 (9), 3723–3750.

ARTICLE

(4) Zhang, L.; Wang, L. Recent research progress on optical limiting property of materials based on phthalocyanine, its derivatives, and carbon nanotubes. J. Mater. Sci. 2008, 43, 5692–5707. (5) de la Torre, G.; Bottari, G.; Hahn, U.; Torres, T. Functional phthalocyanines: Synthesis, nanostructuration, and electro-optical applications. In Structure and Bonding (Berlin); Jiang, J.-Zh., Ed.; SpringerVerlag: Berlin Heidelberg, 2010; Vol. 135, pp 1
44. (6) Liu, Y.; Xu, Y.; Zhu, D.; Wada, T.; Sasabe, H.; Zhao, X.; Xie, X. Optical second-harmonic generation from Langmuir
Blodgett films of an asymmetrically substituted phthalocyanine. J. Phys. Chem. 1995, 99 (18), 6957–6960. (7) Cheng, X.; Yao, S.; Li, Ch.; Manaka, T.; Iwamoto, M. Second harmonic generation at the interface of copper tetra-tert-butyl phthalocyanine Langmuir
Blodgett film/metal. Sci. China, Ser. B: Chem. 2003, 46 (4), 379–386. (8) Nitschke, Ch.; O’Flaherty, S. M.; Kr€oll, M.; Blau, W. J. Material investigations and optical properties of phthalocyanine nanoparticles J. Phys. Chem. B 2003, 107 (44), 12105–12112. (9) Kirichenko, N. A.; Blinov, L. M. Stark spectroscopy of phthalocyanines. Concentration dependences of spectra of metal-free phthalocyanine and copper phthalocyanine. Zh. Prikl. Spektrosk. 1977, 26 (6), 1041–1046(English translation provided by http://www.springerlink. com). (10) Ostuni, R.; Larciprete, M. C.; Leahu, G.; Belardini, A.; Sibilia, C.; Bertolotti, M. Optical limiting behavior of zinc phthalocyanines in polymeric matrix. J. Appl. Phys. 2007, 101, 033116-1–033116-5. (11) Larciprete, M. C.; Mangialardo, S.; Belardini, A.; Sibilia, C.; Bertolotti, M. Realization and characterization of tetra-tert-butyl-zincphthalocyanine
poly(methyl methacrylate) films for optical limiting applications. J. Appl. Phys. 2008, 104, 073109–1
073109
4. (12) Brynda, E.; Kaldova, L.; Koropecky, I.; Nespurek, S.; Rakusan, J. Copper tetra-4-t-butyl-phthalocyanine Langmuir
Blodgett films: photoelectric and structural studies. Synth. Met. 1990, 37 (1
3), 327–333. (13) Sumimoto, M.; Sakaki, Sh.; Matsuzaki, S.; Fujimoto, H. Significant differences in electronic structure among χ-, R- and β-forms of lithium phthalocyanine. Dalton Trans. 2003, 31–33. (14) Ni, Zh.-H.; Li, R.-J.; Jiang, J.-Zh. New progress in monomeric phthalocyanine chemistry: Synthesis, crystal structures and properties. In Structure and Bonding (Berlin); Wu, X.-T., Ed.; Springer-Verlag: Berlin Heidelberg, 2009; Vol. 133, pp 121
160. (15) Sharp, J. H.; Lardon, M. Spectroscopic characterization of a new polymorph of metal-free phthalocyanine. J. Phys. Chem. 1968, 72 (9), 3230–3235. (16) Sharp, J. H.; Abkowitz, M. Dimeric structure of a copper phthalocyanine polymorph. J. Phys. Chem. 1973, 77 (4), 477–481. (17) Perovic, A. Transmission electron microscopic observation of dislocations in χ phthalocyanine crystals. J. Mater. Sci. 1987, 22, 835–838. (18) Zugenmaier, P.; Bluhm, T. L.; Deslandes, Y.; Orts, W. J.; Hamer, G. K. Diffraction studies on metal free phthalocyanines (β-H2Pc and χ-H2Pc). J. Mater. Sci. 1997, 32, 5561–5568. (19) Brinkmann, M.; Turek, P.; Andre, J.-J. EPR study of the χ, R and β structures of lithium phthalocyanine. J. Mater. Chem. 1998, 8 (3), 675–685. (20) Fryer, J. R.; McConnell, C. M.; Hann, R. A.; Eyres, B. L.; Gupta, S. K. The structure of some Langmuir
Blodgett films. Part I. Substituted phthalocyanines. Phil. Mag. B 1990, 61 (5), 843–852. (21) Isago, H. Spectral properties of a novel antimony(III)
phthalocyanine complex that behaves like J-aggregates in non-aqueous media. Chem. Commun. 2003, 1864–1865. (22) Treacher, K. E.; Clarkson, G. J.; McKeown, N. B. Stable glass formation by a hexagonal ordered columnar mesophase of a low molar mass phthalocyanine derivative. Liq. Cryst. 1995, 19 (6), 887–889. (23) Osburn, E. J.; Chau, L.-K.; Chen, S.-Y.; Collins, N.; O’Brien, D. F.; Armstrong, N. R. Novel amphiphilic phthalocyanines: Formation of Langmuir
Blodgett and cast thin films. Langmuir 1996, 12 (20), 4784–4796. 17950

dx.doi.org/10.1021/jp206072m |J. Phys. Chem. C 2011, 115, 17945–17951

The Journal of Physical Chemistry C (24) Kumaran, N.; Veneman, P. A.; Minch, B. A.; Mudalige, A.; Pemberton, J. E.; O’Brien, D. F.; Armstrong, N. R. Self-organized thin films of hydrogen-bonded phthalocyanines: Characterization of structure and electrical properties on nanometer length scales. Chem. Mater. 2010, 22 (8), 2491–2501. (25) Smolenyak, P.; Peterson, R.; Nebesny, K.; T€orker, M.; O’Brien, D.; Armstrong, N. R. Highly ordered thin films of octasubstituted phthalocyanines. J. Am. Chem. Soc. 1999, 121 (37), 8628–8636. (26) Donley, C. L.; Xia, W.; Minch, B. A.; Zangmeister, R. A. P.; Drager, A. S.; Nebesny, K.; O’Brien, D.; Armstrong, N. R. Thin films of polymerized rod-like phthalocyanine aggregates. Langmuir 2003, 19 (16), 6512–6522. (27) Cherian, S.; Donley, C.; Mathine, D.; LaRussa, L.; Xia, W.; Armstrong, N. Effects of field dependent mobility and contact barriers on liquid crystalline phthalocyanine organic transistors. J. Appl. Phys. 2004, 96 (10), 5638–5643. (28) Smolenyak, P.; Osburn, E. J.; Chen, S.-Y.; Chau, L.-K.; O’Brien, D.; Armstrong, N. R. Spectroscopic and electrochemical characterization of Langmuir
Blodgett films of (2,3,9,10,16,17,23,24-octakis((2-benzyloxy)ethoxy)phthalocyaninato)copper and its metal-free analogue. Langmuir 1997, 13 (24), 6568–6576. (29) Drager, A. S.; Zangmeister, R. A. P.; Armstrong, N. R.; O’Brien, D. One-dimensional polymers of octasubstituted phthalocyanines. J. Am. Chem. Soc. 2001, 123 (15), 3595–3596. (30) Pyatosin, V. E.; Tsvirko, M. P. Triplet
triplet absorption spectra of phthalocyanine and its metal complexes. Zh. Prikl. Spektrosk. 1980, 33 (2), 320–325(English translation provided by http://www. springerlink.com). (31) Wang, X.; Li, L.; Ye, H.; Yang, K.; Wang, Ya.-P.; Sandman, D. J. Fabrication of micro- and nanoscale crystals and thin films of a phthalocyanine. J. Macromol. Sci. A 2007, 44 (12), 1323–1327. (32) Valkova, L.; Borovkov, N.; Pisani, M.; Rustichelli, F. Structure of monolayers of copper tetra(3-nitro-5-tert-butyl)phthalocyanine at the air
water interface. Langmuir 2001, 17 (12), 3639–3642. (33) Kanezaki, E. Electronic states of zinc phthalocyanine in various films; its association profile. J. Coord. Chem. 1988, 18, 113–120. (34) B€ottger, B.; Schindewolf, U.; M€obius, D.; Avila, J. L.; Martin, M. T.; Rodriguez-Amaro, R. Preparation and polymorphism of thin films of unsubstituted cobalt phthalocyanine. Langmuir 1998, 14 (18), 5188–5194. (35) Endo, A.; Matsumoto, S.; Mizuguchi, J. Interpretation of the near-infrared absorption of magnesium phthalocyanine complexes in terms of exciton coupling effects. J. Phys. Chem. A 1999, 103 (41), 8193–8199. (36) Schmidt, G.; Sommerauer, M.; Geier, M.; Hanack, M. Synthesis and chromatographic separation of tetrasubstituted and unsymmetrically substituted phthalocyanines. In Phthalocyanines: Properties and Applications; Leznoff, C. C., Lever, A. B. P., Eds.; VCH Publishers: New York, 1996; Vol. 4, pp 5
18. (37) Deng, Z. T.; Guo, H. M.; Guo, W.; Gao, L.; Cheng, Z. H.; Shi, D. X.; Gao, H.-J. Structural properties of tetra-tert-butyl zinc(II) phthalocyanine isomers on a Au(111) surface. J. Phys. Chem. C 2009, 113 (26), 11223–11227. (38) Hassan, B. M.; Li, H.; McKeown, N. B. The control of molecular self-association in spin-coated films of substituted phthalocyanines. J. Mater. Chem. 2000, 10, 39–45. (39) Sommerauer, M.; Rager, Ch.; Hanack, M. Separation of 2(3),9(10),16(17),23(24)-tetrasubstituted phthalocyanines with newly developed HPLC phases. J. Am. Chem. Soc. 1996, 118 (42), 10085–10093. (40) Padma, N.; Joshi, A.; Singh, A.; Deshpande, S. K.; Aswal, D. K.; Gupta, S. K.; Yakhmi, J. V. NO2 sensors with room temperature operation and long term stability using copper phthalocyanine thin films. Sens. Actuators B 2009, 143, 246–252. (41) Wang, Ch.-Yu; Cho, Ch.-P.; Perng, Ts.-P. Structural transformation and crystallization of amorphous copper phthalocyanine nanostructures. Thin Solid Films 2010, 518, 6720–6728. (42) Lee, Yu.-L.; Chen, Yu.-Ch.; Chang, Ch.-H.; Yang, Yu-M.; Maa, J.-R. Surface characterization of the monolayer and Langmuir
Blodgett

ARTICLE

films of tetra-tert-butyl-copper phthalocyanine. Thin Solid Films 2000, 370, 278–284. (43) Turek, P.; Petit, P.; Andre, J.-J.; Simon, J.; Even, R.; Boudjema, B.; Guillaud, G.; Maitrot, M. A new series of molecular semiconductors: Phthalocyanine radicals. J. Phys. Chem. 1987, 109 (17), 5119–5122. (44) Nevin, W. A.; Liu, W.; Lever, A. B. P. Dimerization of mononuclear and binuclear cobalt phthalocyanines. Can. J. Chem. 1987, 65, 855–858. (45) Schutte, W. J.; Sluyters-Rehbach, M.; Sluyters, J. H. Aggregation of an octasubstituted phthalocyanine in dodecane solution. J. Phys. Chem. 1993, 97 (22), 6069–6073. (46) Iwatsu, F. Size effects on the R
β transformation of phthalocyanine crystals. J. Phys. Chem. 1988, 92 (6), 1678–1681. (47) Azumi, R.; Matsumoto, M.; Kuroda, Sh.; King, L. G.; Crossley, M. J. Orientation change of dimer-type porphyrin in Langmuir
Blodgett films caused by a trigger molecule. Langmuir 1995, 11 (10), 4056–4060. (48) Kamanina, N. V.; Denisyuk, I. Yu. Optical limiting of near-IR radiation in a system based on magnesium phthalocyanine. Opt. Spectrosc. 2004, 96 (1), 77–80. (49) Valkova, L.; Borovkov, N.; Kopranenkov, V.; Pisani, M.; Bossi, M.; Rustichelli, F. Some features of the molecular assembly of copper porphyrazines. Mater. Sci. Eng., C 2002, 22, 167–170. (50) Orti, E.; Bredas, J. L.; Clarisse, C. Electronic structure of phthalocyanines: Theoretical investigation of the optical properties of phthalocyanine monomers, dimers, and crystals. J. Chem. Phys. 1990, 92 (2), 1228–1235.

17951

dx.doi.org/10.1021/jp206072m |J. Phys. Chem. C 2011, 115, 17945–17951