Journal of Materials Chemistry C

Halogen bonding, a noncovalent interaction possessing several unique features compared to the more familiar hydrogen bonding, is emerging as a powerful tool in functional materials design. Herein, we unambiguously show that one of these characteristic features, namely high directionality, renders halogen bonding the interaction of choice when developing azobenzene-containing supramolecular polymers for light-induced surface patterning. The study is conducted by using an extensive library of azobenzene molecules that di ﬀ er only in terms of the bond-donor unit. We introduce a new tetra ﬂ uorophenol-containing azobenzene photoswitch capable of forming strong hydrogen bonds, and show that an iodoethynyl-containing azobenzene comes out on top of the supramolecular hierarchy to provide unprecedented photoinduced surface patterning e ﬃ ciency. Speci ﬁ cally, the iodoethynyl motif seems highly promising in future development of polymeric optical and photoactive materials driven by halogen bonding.


Introduction
The azobenzene moiety is an important tool in present-day research of molecular-level photoswitching and is extensively used for the development of macroscopic photonic and photomechanical materials. 1 The interest in azobenzene lies in its clean, efficient, and reversible photoisomerization between a rod-like trans-form and a bent cis-form, 2 which can give rise to a cascade of motions beyond the size scale of an individual molecule. When the azobenzene is incorporated into a liquidcrystalline or a polymeric medium, the photoisomerization can accomplish various tasks, allowing for induction of orderdisorder transitions, control over the molecular alignment, photoactuation of the shape and dimensions of the material system, and inscription of surface-relief patterns onto thin azobenzene-containing lms. 3 In particular, when an azobenzene-containing thin polymer lm is irradiated with a polarization/intensity interference pattern of light, the lm surface deforms and creates a replica of the incident interference pattern in the form of a surface topography pattern, oen denoted as a surface-relief grating (SRG). 4 The modulation depth of the photoinduced surface patterns can be hundreds of nanometers, creating efficient diffractive optical elements with a rst-order diffraction efficiency of up to 30-40%. SRGs have been used as nanostructuring tools, liquid-crystal alignment layers, and periodic light trapping/light outcoupling layers. 5 Despite their application potential, however, the surface pattern formation mechanism is not yet well understood. Therefore, a full understanding of structure-function relationships that govern the SRG process is needed in order to design better performing materials for future advanced applications. Supramolecular side-chain polymers, in which the photoactive azobenzene units are attached to a passive polymer backbone via noncovalent interactions, have in the past few years emerged as a new class of SRG-forming materials. 6 The importance of these materials lies in their extreme ease of preparation and modular tunability: the material composition can be easily optimized to meet specic requirements. As a result, high-quality resonance lters, and patterns with recordhigh modulation depth have been fabricated from azobenzenecontaining supramolecular side-chain polymers. 6a,7 We have recently shown that halogen bonding 8 appears particularly promising for the design of supramolecular SRG-forming materials. 9 Halogen bonding is the noncovalent interaction in which a region of positive electrostatic potential on top of the surface of the halogen atom, the s-hole, 10 interacts with a nucleophilic site. High-level ab initio quantum-chemical calculations suggest that dispersion forces may also play an important role in this interaction. 11 The features that distinguish halogen bonding from hydrogen bonding are of particular interest, which include higher directionality, hydrophobicity, size of the interacting atom (halogen vs. hydrogen) and the possibility to tune interaction strength by a simple change of the halogen atom. 12 These characteristic properties have allowed for the ne tuning of geometrical features relevant to crystal engineering 13 and to the design of functional materials with unique light-emissive 14 and liquid-crystalline 15 properties.
Recently several examples of halogen-bonded supramolecular structures involving azobenzenes have been reported. 9,16 For example, we showed that the efficiency of SRG formation increases with the strength of halogen bonding, and that the high performance of halogen-bonded complexes seems to be connected more to the directionality of halogen bonding than to its intrinsic interaction strength. To further develop the practical applications of halogen bonding in the eld of photocontrollable materials, we have undertaken a detailed study on the relative surface patterning efficiency that can be imparted by different halogen-bond donors as a result of the presence of electron-withdrawing substituents versus the hybridization of the carbon bearing the heavy halogen. Moreover, we aimed to provide a ranking between various halogen and hydrogen bonds in driving SRG formation, since these two interactions show similar chemical behaviour and a detailed knowledge of the performance of competing noncovalent interactions is crucial for the preparation of materials with designed structures and properties.
Herein, we present a comprehensive study on the surface patterning properties of an extensive library of azobenzene derivatives as presented in Fig. 1. We show that halogen-bonded polymer-azobenzene complexes unambiguously surpass their hydrogen-bonded counterparts in terms of SRG formation efficiency, conrming that it is the directionality rather than the strength of the noncovalent interactions that dictates the macroscopic light-induced movements. Most importantly, we introduce in this eld a new halogen-bond donor moietyiodoacetylenethat further boosts the macroscopic lightinduced movements and the surface pattern formation. The optical studies are complemented with theoretical calculations and crystallographic structure determinations that further elucidate the different performance of halogen-and hydrogenbonded systems.

Materials design
The library of azobenzene compounds shown in Fig. 1 allows for (i) establishing an optimal halogen-bond donor in terms of SRG formation capability, and (ii) comparing structurally similar azobenzenes substituted with halogen-bond versus hydrogenbond donor groups. The series of peruorinated azobenzenes 1-5 comprises a reference molecule incapable of forming halogen bonds (1), two halogen-bond donors (2, 3) with similar spectral and photochemical properties but different interaction strengths (Br < I), and two hydrogen-bond donors, one weak (4) and one strong peruorophenol-containing dye (5). Compound 5 serves a dual purpose. Firstly, a close structural relationship with 3 affords an ideal comparison for halogen bonding versus hydrogen bonding in driving the SRG formation process. Secondly, the peruorinated phenol presumably acts as a very powerful hydrogen-bond donor and could therefore give rise to a very strong noncovalent interaction, 17,18 which is interesting in its own right in the eld of functional supramolecular materials. This is the rationale for the comparison between 5 and its nonuorinated counterpart 7, the SRG performance of which is known. 9 The non-uorinated dyes 6-9 include the already mentioned phenol-substituted compound 7, an iodo-substituted azobenzene 6 that is expected to act as a weak halogen-bond donor, and ethynyl-substituted dyes 8 and 9. The latter two compounds are particularly interesting, since very recently iodoethynylbenzenes were investigated both theoretically and experimentally with the purpose of establishing a hierarchy among halogen-bond donors in crystal engineering. 19 Based on ATR-FTIR and single crystal X-ray diffraction data, Aakeröy et al. proved that the iodoethynyl benzenes form co-crystals with a larger class of halogen-bond acceptors compared to per-uoroiodobenzenes suggesting that this motif might be employed to enhance the performance of photoresponsive polymers. Each of the studied molecules is substituted with an electron-donating dimethylamino group, which red-shis the absorption spectrum of the dyes, increases the rate of trans-cistrans cycling, and boosts the SRG formation efficiency.
Poly(4-vinyl pyridine) (P4VP) was used as a polymeric bond acceptor as it is a popular choice in constructing supramolecular polymeric complexes based on hydrogen bonding or proton transfer, 20 and it equally functions as a halogen-bond acceptor. 21 P4VP has been used by several groups to prepare light-responsive polymers, 6b,22 making it the polymer of choice for the present study. Low-molecular-weight P4VP (M w : 1200 g mol À1 ) was selected since long polymer chains tend to disrupt the photoinduced surface patterning. SRG formation was studied in polymer-azobenzene complexes, denoted as P4VP(n) y where n is the azobenzene unit chosen among 1-9 and y is the n : P4VP molar ratio. Typically, y was set to be 0.1, or, in other words, the studied systems contained one azobenzene moiety per ten polymer repeat units. This low degree of complexation minimizes crystallization and macroscopic phase separation of the azobenzene units, which would compromise comparisons between the molecules. Assuming that (E)-1,2di(4-pyridyl)ethylene (10) is a good mimic of the minimal binding motif of P4VP, the co-crystals formed by 10 with various azobenzene derivatives were used as models for single-crystal Xray diffraction analysis of the P4VP-azobenzene systems employed in light-induced surface patterning. 4-Methyl pyridine was used as a model of the minimal binding motif of P4VP in computational studies.

Computational studies
As mentioned above, 4-methyl pyridine was used as the smallest pyridine model compound mimicking P4VP to compute the binding energy with azobenzenes 1-9. Table 1 presents the PBE0/6-311++G(d,p) interaction energies, both non-corrected (DE) and corrected (DE BSSE ) for the basis set superposition error, showing a notable energy overestimation (from 6 to 14% depending on the dimer) when neglecting such an error. The DE BSSE values computed for the peruorinated dyes 2 and 3 were À3.501 and À5.135 kcal mol À1 , 9 respectively, in agreement with the consolidated trend of Br < I in halogen-bonded systems, 10 while molecule 1, lacking the s-hole on the uorine atom in the para position to the azo group, was conrmed to be unable to give rise to a stable halogen-bonded complex. For the complex of 4-methyl pyridine with the peruorinated hydrogenbond donor 5, a stabilization energy of À11.765 kcal mol À1 was obtained, corresponding to a medium-to-strong hydrogen bond. 23 The corresponding iodinated and phenolic non-uorinated donors, 6 and 7, provided stabilization energy values of À2.546 and À10.053 kcal mol À1 , respectively, upon binding to the pyridine unit.
The key observation here is that peruorination of the benzene ring is signicantly more effective in strengthening halogen bonding than hydrogen bonding: the interaction strength, in fact, doubles in the rst case (6 vs. 3), whereas a minor <20% enhancement takes place for the latter (7 vs. 5). This difference can be rationalized by examining the changes in charge density distribution around iodine and the phenolic hydrogen in the isolated dyes upon uorination. While the molecular dipole moment (Table 1, m calc ) undergoes a greater increase for the phenolic with respect to the iodinated derivative, the electrostatic potential ( Fig. 2) is practically unaffected by uorination near the phenolic hydrogens, though it is strongly positive in both cases (maximum values of V S,max are 0.083 and 0.086 au for 7 and 5, respectively, on the 0.001 a.u. isosurface of electron density). Conversely, the maximum value of the electrostatic potential around the iodine atom, while less positive, doubles upon ring uorination, from 0.018 a.u. (6) to 0.035 au (3). This can be explained by polarizability arguments: it is well-known that halogen atomsapart from uorineare highly polarizable, and more so for the heavier halogens. As a result, the strength of halogen bonding signicantly increases by introduction of electron-withdrawing groups such as uorine atoms on the molecular moiety bonded to the halogen-bond donor site. 24 As recently shown by a systematic investigation of a series of halogen-bond donors, 19 the hybridization of the carbon atom adjacent to the halogen bonded atom heavily inuences the strength of the formed halogen bonds. In particular, iodoethynyl-containing moieties have recently emerged as remarkably powerful halogen-bond donor moieties. 19 In agreement with such ndings, compound 8 shows, upon binding to the model pyridine unit, a slightly higher stabilization energy (À5.170 kcal mol À1 ) than compound 3. It is also noteworthy that Fig. 1 Chemical structures of fluorinated azobenzene modules 1-5, hydrogenated azobenzene modules 6-9, model dipyridyl compound 10 used in the co-crystallization studies, and poly(4-vinyl pyridine) (P4VP) that was used as the polymer matrix for the azobenzene dyes. Table 1 Non-corrected and corrected interaction energies of compounds 1-9 with 4-methyl pyridine (DE and DE BSSE , respectively, kcal mol À1 ), local most positive values of electrostatic potential computed on an isosurface (0.001 a.u.) of electron density (V S,max , a.u.) and ground-state dipole moments (m calc , D) of 1-9. Interaction energies and V S,max values were computed in vacuo and dipole moments in DMF. All properties were evaluated at the PBE0/6-311++G(d,p) level of theory the maximum value of the electrostatic potential around the iodine atom is 0.041 a.u., notably larger than the corresponding value obtained for 3 (0.035 a.u.), suggesting that the iodoethynyl moiety should give a more directional halogen bonding. Such observation prompted us to check the different directionality features of the corresponding hydrogenated derivatives, 9 vs. 4. While the hydrogen bond of the former with 4methyl pyridine is slightly weaker than that of the latter (stabilization energies are À3.528 and À3.930 kcal mol À1 , respectively), the ethynyl derivative is characterized by a highly sharpened positive region in the electrostatic potential (Fig. 2), with a more positive V S,max value with respect to compound 4 (0.043 and 0.039 a.u., respectively). This suggests a potentially much greater directionality for the hydrogen bonding of the ethynyl derivative with respect to the peruorinated compound, somewhat resembling halogen bonding.
It is thus clear that 4-methyl pyridine is more tightly bound to the phenolic and hydrogen-bonding donor dyes 5 and 7 than to the halogen-bonding donor dyes 3 and 6, but it is also evident from Fig. 2 that halogen bonds are expected to be signicantly more directional, 25 as the positive region responsible for the noncovalent interaction is narrowly conned along the extension of the C-I bonds and it is spread out hemispherically around the phenolic hydrogens. On the other hand, the dimeric complexes comprising 4-methyl pyridine and ethynyl derivatives 8 and 9 show quite similar directionality features but the interaction strength of the iodine derivative is greater. In our hypothesis, a more directional noncovalent interaction between the azobenzene dye and the P4VP provides a more efficient junction in transferring the light-induced motions of the photoactive units to the polymer backbone and enhanced SRG formation results. 9 Molecules 3 and 5 comprise an ideal pair to gain further insight into this hypothesis.
We also computed the UV-visible spectra for 1-9 by timedependent DFT calculations (Table S1 †), which showed that peruorination of the benzene ring red-shis the absorption maxima of both iodine and phenolic derivatives, as was also veried experimentally by measuring the absorption spectra in DMF solutions. Moreover, the spectroscopic properties of the tetrauoroiodo and iodoethynyl derivatives 3 and 8, respectively, are quite similar indicating that the two halogen-bond donors share many parallel features. Such ndings are extremely helpful for the design of new functional materials based on halogenated ethynyl moieties. Details on computations are given in the ESI. †

Structural characterization
(E)-1,2-Di(4-pyridyl)ethylene (10) was used as a co-crystal forming component that mimics P4VP. The aim was to get specic insight into the binding between the pyridine moiety and the azobenzene derivatives 5 and 8 which are new dyes introduced here. These two molecules were co-crystallized with the bispyridine derivative 10 in a 2 : 1 ratio.  Good-quality single crystals of the complex 10$(5) 2 were obtained by slow diffusion of diethyl ether into a methanol solution containing the two starting compounds at room temperature. X-ray diffraction analysis shows that each molecule of 10 binds two different phenol dyes 5 via short N/H hydrogen bonds (Fig. 3a). The O/N distance is 2.557(2)Å, the C-O/N angle is 125.68(3) and a sigmoidal trimeric unit is formed. The observed hydrogen bond is among the shortest found in the Cambridge Structural Database (CSD) and is comparable to that found in the pentachlorophenol/4-methylpyridine co-crystal wherein the O/N distance is 2.515Å. 26 The O-H/N hydrogen bond present in the co-crystal 10$(5) 2 is shorter than that found in a similar co-crystal between the nonuorinated analogue 7 and 1,2-di-pyridylethane (O/N distance 2.703Å). 9 This difference in hydrogen bond distances of co-crystals formed by the uorinated and hydrogenated azophenols 5 and 7 parallels very well the difference in interaction energies from theoretical calculations (Table 1), i.e., the enhanced electron density acceptor capability of the phenolic hydrogen in 5, leading to a shorter and stronger hydrogen bond than in 7, is consistently proved by experimental and computational studies.
In the three-dimensional crystal packing, the 10$(5) 2 trimers form well-dened anti-parallel dimers due to an off-set stacking of the tetrauorobenzene rings (the centroid-centroid distance is 3.445Å) (Fig. 3b). 27 Further p/p stacking involves the tet-rauorobenzene rings and the dimethylaminobenzene rings of adjacent dimers or trimers (the centroid-centroid distance is 3.684Å). 28 Such stacking interactions, while useful in crystal engineering 29 as well as in liquid-crystal self-assembly, 30 may cause dye aggregation within polymer matrices, thus preventing high doping concentrations that would be benecial in optoelectronic applications.
The co-crystal formed between 8 and 10 is a strong example of the high directionality of halogen bonding. Single crystals of 10$(8) 2 were obtained by slow evaporation at room temperature of a THF solution containing the two starting modules. The single crystal analysis revealed the formation of rod-like halogen-bonded trimers in which a 1,2-di(4-pyridyl)ethylene molecule bridges two iodoethynyl azo-dyes (Fig. 3c) by short and linear halogen bonds with N/I distance of 2.754(3)Å and C-I/N angle of 175.29 (10) . The disorder present in the structure prevents the possible further aggregation of these trimers to be analysed. The structural characterization of the model co-crystals highlights that both of the new dyes 5 and 8 interact strongly with pyridine-bearing entities; therefore their incorporation into the P4VP matrix may lead to the formation of new supramolecular polymeric materials with efficient photoswitching functionalities. Table 2 shows the absorption maxima, relative decreases in absorbance upon irradiation (488 nm, 50 mW cm À2 , circular polarization), and time constant estimates of the cis-trans thermal isomerization of thin lms of the supramolecular complexes between P4VP and dyes 1-9. The absorbance decrease relates to the amount of cis-isomers in the photostationary state. The thermal cis-trans relaxation is a known non-monoexponential process in heterogeneous polymer environments and tting the relaxation curves as a sum of rst-order processes lacks physical justication. 31 Hence the relaxation time constants, which are obtained using reported procedures, 32 are only approximate and are given to facilitate the comparison among compounds of the molecular library of Fig. 1.

Photochemical studies
Apart from compounds 6 and 7, the absorption maxima of the molecules studied are very similar. Fluorination of the bond-donor ring red-shis the l max by 25 nm for the iodosubstituted azobenzenes (6 vs. 3) and 48 nm upon phenolsubstitution (7 vs. 5). The ethynyl group plays a similar, though not as pronounced, role (6 vs. 8), due to an increase in the Hückel conjugation length. The absorbance decrease upon illumination is very similar, around 30% for compounds 2-6, and around 20% for 1 and 7. Due to similar trans-cis isomerization efficiencies, the photoinduced surface patterning may be compared well with each other within our set of molecules. Interestingly, the photostationary state is much more cis-rich for the ethynyl-substituted molecules 8 and 9, and the lifetime of the cis-isomer is longer, possibly because of the larger aspect ratio of these molecules as compared to those of 1-7.
To summarize, the compounds shown in Fig. 1 cover the range from weak to strong halogen and hydrogen bond donors and the fact that a central core providing similar absorption properties can be maintained allows for facile and valid comparisons between these dyes in the same polymer matrix.

Supramolecular hierarchy in SRG formation
The molecular library presented in Fig. 1 allows us to explore various aspects of the surface patterning process, particularly to address the role of (i) the interaction strength, (ii) halogen versus hydrogen bonding, and (iii) the ethynyl group in the efficiency of the SRG inscription. Such a comprehensive comparison is only valid when using a polymer host of low polydispersity (we used P4VP with M w ¼ 1200 g mol À1 ; M w /M n ¼ 1.2) containing equal amounts of noncovalently attached azobenzene units (10% mol mol À1 in this case), and lms of equal thickness (150 AE 10 nm).
The best way to study the efficiency of SRG formation is to monitor the rst-order diffraction generated by periodic modulation of the sample surface due to interference irradiation. Several simultaneously formed and coupled gratings contribute to the overall diffraction efficiency, 33 but in thin amorphous azo-polymer lms, by far the largest contribution arises from the SRG formation. Hence, we can relate the efficiency of the SRG formation to the growth dynamics of the diffracted signal. All samples provided high-quality sinusoidal gratings with a well-dened modulation depth ranging from 30 to 180 nm ( Table 2).
The role of halogen bonding strength. The kinetics of the diffraction efficiency of polymer-azobenzene complexes (in which the azobenzene unit is attached to every 10th polymer repeat unit) containing molecules 1-3 is given in Fig. 4a. A clear trend is seen: the non-halogen-bonded reference molecule 1 yields very weak diffraction, whereas the diffraction efficiency of the halogen-bonded complexes based on 2 and 3 develops in the order I > Br, i.e., a stronger interaction signicantly boosts the SRG formation efficiency. The same trend can be seen in the AFM surface proles shown in Fig. 4b, where the surface modulation develops in the order I > Br > F. For the iodinated complex P4VP(3) 0.1 the modulation depth is ca. 175 nm (see Table 2), exceeding somewhat the thickness of the polymer lm (150 nm). This result is in line with our earlier observations 9 and shows that (i) interaction between the photoactive azobenzene units and the passive polymer host is essential for the lightinduced mass transport to take place, and (ii) if the azobenzenes are similar in terms of their spectroscopic and photochemical properties (as is the case of 2 and 3), the higher the interaction strength the more efficient the mass transport.
Halogen bonding versus hydrogen bonding. Molecules 3 and 5, together with their nonuorinated counterparts 6 and 7, provide a comprehensive set for comparing the SRG formation efficiency in halogen-and hydrogen-bonded complexes, allowing us to assess the role of interaction directionality in the SRG formation. We recall that the interaction strength with pyridine moieties develops in the order 5 (11.765 kcal mol À1 ) > 7 (10.052 kcal mol À1 ) > 3 (5.135 kcal mol À1 ) > 6 (2.546 kcal mol À1 ), being signicantly higher for the hydrogen-bonded complexes. The absorption maxima of 3 and 5 are practically identical as is the absorbance decrease upon irradiation (see Table 2). The latter relates to the efficiency of trans-cis photoisomerization, and to the amount of cis-isomers in the photostationary state. The cistrans thermal relaxation is somewhat faster for P4VP(5) 0.1 (lifetime of ca. 4 min vs. ca. 10 min) but as the 488 nm irradiation that is used for SRG inscription triggers both trans-cis and cistrans photoisomerization, we believe that the thermal isomerization has a negligible effect on the overall trans-cis-trans cycling rate. Therefore, we argue that the differences in thermal isomerization rate do not signicantly affect the SRG formation and if they do have a small contribution, it will be in favour of 5.
The diffraction efficiency curves for the complexes between P4VP and 3, 5-7 are presented in Fig. 5. As expected, based on Fig. 4, for the halogen-bonded complexes the diffraction efficiency increases with increasing interaction strength; hence uorination of the iodobenzene ring of 6 to afford 3 greatly enhances the SRG formation. In the systems based on azophenols 5 and 7, the diffraction kinetics is rather similar but the diffraction efficiency is systematically higher for the uorinated one P4VP(5) 0.1 . The surface-modulation depths aer 30 min inscription are ca. 170 nm and ca. 130 nm for P4VP(5) 0.1 and P4VP(7) 0.1 , respectively. The difference is likely due to slightly higher interaction strength and more conned electropositive surface of the former.
The most interesting comparison is that between P4VP(3) 0.1 and P4VP(5) 0.1 , that is, between similar halogen-bonded and Fig. 4 (a) Kinetics of the first-order diffraction efficiency upon the grating inscription process, and (b) AFM surface profiles of the gratings for P4VP(n) 0.1 , where n ¼ 1 (F is para to the azo group), 2 (Br is para to the azo group), and 3 (I is para to the azo group).
hydrogen-bonded complexes. Even if the eventual diffraction efficiency and the modulation depth are practically the same, the dynamics of the diffraction growth is signicantly faster for the halogen-bonded complex, i.e., the SRG formation efficiency is higher for the iodinated dye despite its much weaker interaction with the polymer. As their chemical structures are identical apart from the bond-donor group, and the spectral and photochemical properties are comparable, this comparison unambiguously shows that, when sufficiently strong, halogen bonding surpasses hydrogen bonding in driving the lightinduced mass transport in polymer-azobenzene complexes, due to its higher directionality. The order of the efficiency is reversed only when halogen bonding is signicantly weaker than hydrogen bonding.
The role of the ethynyl group. Noncovalent interactions mediated by acetylene groups have not been previously used in light-induced surface patterning, and iodoacetylenes have only recently emerged as supramolecular synthons in crystal engineering and topochemical polymerization. 19,34 Hence it is of great interest to compare molecules 3 and 8, to address the question whether or not iodoacetylene is better than iodoper-uorobenzene in driving the SRG formation process. In this respect, it is also interesting to compare the SRG formation in complexes containing molecules 4 and 9. Their interaction energy with pyridine moieties is approximately the same (Table  1), but the molecular shape in the region of the bond donor moiety and the associated charge distribution are completely different, giving rise, in 9, to an electrostatic potential surface around the hydrogen atom that is very similar to that seen in the halogenated derivatives (Fig. 2). The positive area is narrowly conned along the extension of the C-H bond, closely resembling the s-hole in halogens, surrounded by a belt of negative potential. Hence, this comparison allows us to study whether the more directional (weak) hydrogen bond leads to a more efficient mass transport.
The diffraction curves are shown in Fig. 6, and based on them we can draw two important conclusions. First, among the nine supramolecular systems studied, the iodoethynyl-based P4VP(8) 0.1 complex is the most efficient for SRG formation. The difference with P4VP(3) 0.1 is not large, but it is systematic and repeatable. We believe that such enhancement results from more favourable photochemical properties, and possibly larger free-volume sweep upon isomerization, which may enhance the SRG formation. 35 Note also that the longer cis-lifetime does not seem to hinder the mass transport process as we suggested earlier. The fact that we can maintain efficient optical performance while removing the uorine atoms from the bond-donor ring may be a great advantage when designing polymeric optical materials based on halogen bonding. The reduced quadrupolar interactions between the chromophores probably allow for using higher chromophore loadings without crystallization and phase separation of the dye, which may be benecial in applications that require, e.g., high optical anisotropy or nonlinear optical response.
The second important conclusion relates to the role of directionality of the polymer-azobenzene noncovalent interaction in the SRG formation. The P4VP(9) 0.1 complex performs worse than the halogen-bonded complexes (Fig. 6), but better than P4VP(4) 0.1 . This relative efficiency supports the data of Fig. 5 and suggests that more directional interactions of comparable strength lead to more efficient mass transport. We attribute this trend to formation of electrostatics-controlled rigid polymer-azobenzene junctions that "force" the passive polymer chains to follow the movements of the azobenzene molecules upon isomerization. The role of directionality explains the potential of halogen bonding in this application, and also calls for further investigation into the SRG formation in, e.g., complexes driven by strong and directional multiple hydrogen bonds. 36 Experimental Compounds 2, 3 and 4 were synthesized as previously reported. 9 Dye 7 was purchased from TCI Europe. Synthesis of azobenzenes 1, 5, 6, 8 and 9 is described in detail in the ESI. † Fig. 5 Kinetics of the first-order diffraction efficiency upon the grating inscription process for P4VP(n) 0.1 , where n ¼ 3 (I is para to the azo group), 5 (OH is in the para position), 6 (I is in the para position on a non-fluorinated ring), and 7 (OH is in the para position on a nonfluorinated ring). The molecular structures of dyes 1-9 were optimized in DMF within the DFT approach, using the PBE0 functional, 37 which has been judged to be well suited for describing the electronic and optical features of a series of organic dyes, 38 and treating the energetic and geometric features of halogen bonding, both in vacuo 39 and in a solvent. 40 The 6-311++G(d,p) basis set was used for all atoms. The solvation effects have been included by means of the Polarizable Continuum Model (PCM). 41 to determine the absorption wavelengths, standard vertical time dependent 42 PBE0/6-311++G(d,p) calculations have been carried out. The molecular dimers of compounds 1-9 with 4-methyl pyridine have been optimized at the PBE0/6-311++G** level of theory in vacuo. Interaction energies DE BSSE have been computed by optimization on the BSSE-free potential energy surface as the difference between the energy of the dimer and the sum of the energies of the single monomers. BSSE correction was made by the standard counterpoise method. 43 All calculations have been performed with the Gaussian suite of programs. 44 The single-crystal X-ray structures were determined using a Bruker Kappa Apex II CCD diffractometer with Mo Ka radiation (l ¼ 0.71073Å) and a Bruker Kryoex low-temperature device. Crystals were mounted in inert oil on glass bers. Data collection and reduction were performed by SMART and SAINT and absorption correction, based on the multi-scan procedure, by SADABS. The structures were solved by SIR92 and rened on all independent reections by full-matrix least-squares based on F o 2 by using SHELX-97. Crystallographic data are reported in Table S1. † CIFs containing full crystallographic data can be obtained free of charge from the ESI. † Surface-relief gratings were inscribed on thin lms (150 AE 10 nm as measured with a DEKTAK 6M surface proler) spin-cast from freshly prepared, ltered DMF solutions of the azobenzene-polymer mixtures on quartz substrates. The inscription was performed using a spatially ltered and collimated Ar+ laser beam (Coherent Innova 70) with circular polarization at a wavelength of 488 nm with an irradiation intensity of 150 mW cm À2 . Lloyd's mirror interferometer, with an incidence angle of 15 (spatial period of 1 mm) was used to create the light-interference pattern. The dynamics of the gratings was monitored by measuring the transmitted rst-order diffracted beam from a normally incident 633 nm He-Ne laser. The diffraction efficiency of the gratings was dened as h ¼ I 1 /I 0 , where I 1 and I 0 are the intensities of the rst-order diffracted beam and the transmitted beam prior to irradiation, respectively. The surface proles of the inscribed gratings were characterized using a Veeco Dimension 5000 atomic-force microscope.
The UV-Vis spectra were collected from both thin lms and dilute (10 À5 M) DMF solutions with an Ocean Optics USB2000+ ber-optic spectrometer and a DH-2000-BAL light source, both in the dark and under irradiation (488 nm, 50 mW cm À2 ). Thermal cis-trans isomerization was studied by exciting the chromophores to the cis-state with a circularly polarized pump beam (488 nm, 50 mW cm À2 ) and monitoring the transmittance changes aer blocking the pump. The probe was a ber-coupled xenon lamp equipped with proper bandpass lters. The signal was detected with a photodiode and a lock-in amplier.

Conclusions
We have investigated the formation of photoinduced surface patterns in halogen-bonded and hydrogen-bonded polymerazobenzene supramolecular complexes. Using an extensive molecular library we were able to draw several important conclusions on the light-induced mass transport process. First of all, halogen-bonded complexes outperform the corresponding hydrogen-bonded complexes (cf. Fig. 1, compounds 3 and 5) in terms of surface patterning efficiency, as a result of the higher directionality of halogen bonding. Greater directionality promotes more efficient mass transport also when hydrogen bonding is responsible for the dye-polymer binding. Furthermore, within the halogen-bonded series (cf. molecules 1, 2, 3, and 6) the efficiency increases with the interaction strength. The most efficient halogen-bond donor motif that comes on top of the supramolecular hierarchy in driving SRG formation is the iodoacetylene dye 8. This efficiency is due to its favourable photochemical properties as compared to the tetra-uoroiodobenzene motif. We believe this motif to widen the future perspectives of the design of halogen-bonded optical and photoresponsive materials by allowing for high chromophore loadings to be used in polymer matrices. Research in this direction is currently ongoing in our laboratories and will be reported elsewhere.