Tuning linear dichroism/birefringence (LDLB) contributions in VCD spectra of tartaric acid-TPPS₄ porphyrin films

Abstract

Vibrational circular dichroism (VCD) is employed to investigate chiral-templation of D and L tartaric acids (TA) which, according to electronic circular dichroism (ECD) studies, drive tetrakis-(4-sulfonatophenyl)porphyrin (TPPS₄) to self-assemble into chiral J-aggregates. TPPS₄/TA solutions, prepared in both H₂O and D₂O, are drop cast on CaF₂ plates. VCD spectra of corresponding films are not affected by rotating the samples around the incident beam, but undergo dramatic changes, even sign reversal, by flipping the samples from “front” to “rear” position, depending on acid content. The observed IR absorption features are mainly due to vibrational transitions of TA. The dissymmetry ratio g for the observed features is of the order of 10⁻², which is quite high for vibrational phenomena: the semi-sum spectra in the front-rear positions can be considered as representative of “intrinsic” VCD phenomena, the semi-difference is associated with linear dichroism-linear birefringence components (LDLB term). In this study, we show that TPPS₄ and TA, when mixed appropriately following the specified protocols, yield films in which the VCD and LDLB components can be effectively modulated.

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Introduction

Vibrational circular dichroism (VCD) experiments are usually conducted on solutions mainly to assign the absolute configuration of chiral compounds (either of natural or synthetic origin),^1–4 to investigate conformational properties^5–7 of molecular or supramolecular systems^8,9 or secondary peptide structure.^10,11 VCD has been demonstrated to be a sensitive technique to conformational changes and environment interactions. Use of IR solid state spectroscopy has always been of primary interest, especially by employing KBr pellets.¹² Despite its 50 year history,¹³ the use of VCD for solid state samples is not common^14,15 being prone to artifact. However, in recent years, both CD and VCD spectroscopies in solid state have met significant attention and are catching up rapidly both experimentally and theoretically.^16–21 Particular interest has emerged in the fields of material science^22,23 and quality control,^16,24,25 considering both pellets and films.^26,27 Recently, important advances in solid state VCD calculations have been developed.^28–30 The VCD spectrum of a compound in the solid state may be quite different from the one recorded in solution. In crystal form, molecules are packed in regular lattices and the conformational freedom is quite restricted leading to narrower IR and VCD bands.²⁹ Moreover, interactions with nearby molecules are stronger and can generate supramolecular chiral structures. As a result, solid state VCD can be rich in relevant information. However, it requires particularly careful experimental procedures in order to discriminate effects of different nature. The great interest in the subject is also witnessed by the recent publication of a review article titled “VCD beyond solutions”.³¹

The various contributions affecting VCD in the solid state and its electronic counterpart (ECD), though to a different degree, have been discussed in the literature, where many papers have appeared. We cite here just a few of them, to the risk of being impolite to some Authors, all of them applying the Mueller matrix method,^{13,26,27,32–38} and touching upon most aspects of optical activity of solid state samples, even VCD and films. Without entering in detailed explanations, here we just quickly recall the contributions to an observed spectrum of “so called CD”: (1) circular dichroism, namely absorption difference of the two types of circularly polarized light (CD); (2) linear dichroism, namely absorption difference of the two linearly polarized components at two orthogonal axes (LD and LD′ for two 45°-rotated axes pairs); (3) differences in the refraction indexes of the two circularly polarized components (CB = circular birefringence, otherwise called ORD = optical rotatory dispersion); (4) differences in the refraction indexes of the two linearly polarized components at two orthogonal axes (LB and LB′ = linear birefringence for two 45°-rotated axes pairs). We are mostly interested in the first contribution (CD) and even though LD and LB may be thought of as artifacts —or more precisely, as “disturbances”—in CD studies, their contributions may be exploited for some applications.³⁹ Apart from distortions caused by crystal scattering (Christensen effect),⁴⁰ depending on the size of solid particles/aggregates, LD and LB receive contributions from intrinsic solid medium anisotropy (particles or films) due to linear alignment of organized structures within the sample, leading to significant differences in refraction indices (LB/CB) for orthogonal states or linear polarization.¹³ Salji et al.³⁶ have recently proposed a semiclassical treatment to calculate LD/LB response.

Considering the VCD enhancement often observed in the solid state and the richness of signals typical of the IR spectroscopic range, the use of solid state VCD can be quite informative. In the present work, we aim to contribute to the use of VCD analysis for film characterization. To this instance we have undertaken the study of films obtained from the zwitterionic form H₂TPPS₄ of tetrakis-(4-sulfonatophenyl)porphyrin (TPPS₄) for which the group in Messina has long experience.^41–47 Acidic TPPS₄ are involved in an end-to-end self-association mechanism giving rise to aggregates in solution which exhibit an excitonically coupled spectrum corresponding to an intense UV absorption band at longer wavelength (at about 490 nm) with respect to the corresponding Soret monomer band, known as Jelley-Scheibe (J-band), accompanied by a less intense H-band at shorter wavelength (at about 420 nm).^48,49 J and H components (see Scheme 1) are both present due to the nanotube supramolecular structure, despite the fact that in the literature they are usually indicated as J-aggregates being their spectroscopic signature the one that dominates. Also Q bands, often disregarded being very weak, are indicative of J-aggregate presence, signaled by absorption at 706 nm and correspondingly CD activity (see Fig. S2 right panels). UV and ECD spectroscopies allow to monitor such J-aggregation condition in which the protonated positive core of one molecule has electrostatic interactions with negative sulfonate groups of adjacent molecules (see Scheme 1). We recall that porphyrins have been widely studied focusing on chirality transfer phenomena^50–53 also in aggregated form.^54–56

Scheme 1 Diacid form structure of TPPPS₄, schematics of J-aggregate scheme, and of J–H aggregation within a nanotube.

An interesting property of TPPS₄ system is that it exhibits symmetry breaking phenomenon even in achiral conditions.^41,57,58 Acidic environment contributes to accelerating or controlling the aggregation kinetics and the counter-anions play an important role according to the Hofmeister series. Chiral self-assembling is sensitive to initial conditions such as pH, ionic strength and concentration.^42,44,59 To this instance, in the work by Occhiuto et al.⁴³ TPPS₄ micromolar solutions (3 μM) show an intense positive ECD couplet at 490 nm (+,− while decreasing wavelength), whose intensity is strongly dependent on the ability of counter-anions to interfere with hydrogen-bonding network of the solvent. In this work, we employed millimolar (1 mM) acid TPPS₄ solutions and obtained an opposite chiral behavior: a negative ECD couplet (Fig. S1) centered at 490 nm (however we reproduced the same results of ref.⁴¹ when working at the same micro-molar concentration employed there). A further interesting feature of the formation of TPPS₄ aggregates, already pointed out by Zagami et al.,⁶⁰ is a kinetic effect experienced in D₂O solutions which promote J-aggregation faster than in H₂O solutions.

The mechanism of symmetry breaking in TPPS₄ system, in absence of chiral agents, is still under debate and is beyond the scope of this research; in this work, instead, we exploited TPPS₄ J-aggregates, chirally triggered by addition of solutions of D- and L-tartaric acid enantiomers (TA) whose formation in solution can be easily monitored by ECD-UV.⁴³

Once aggregates are formed, as proven by absorption and ECD spectra (Fig. S2), we used such solutions to deposit drop-cast films on CaF₂ plates and we examined them through VCD. We report the crude experimental results for various film orientations and examine data taken first with the IR beam impinging directly on the film and afterwards data obtained with the light beam incident on the opposite, clean surface of the CaF₂ plate (flipped positions): we call the two positions “front” and “rear” respectively. Once acquired the data, orientation-averages and sum and difference spectra for the “front” and “rear” data are considered. Since observed IR bands are dominated by tartaric acid contributions, we examined also DMSO solutions, KBr-pellets and drop-cast film obtained by water TA solutions. Also, in this case solid state spectra are analyzed in terms of CD, LD and LB contributions.

In fact, in this work it will be shown that linear dichroism-linear birefringence contributions, generally ascribed as “side effects”, are quite reproducible; this point is rather important to apply computational models like those by Salji et al.³⁶ One of the main outcomes of the present analysis is that experimental conditions can be controlled in order to modulate all components to the overall CD spectra that is, “true” CD and LD-LB contributions.

Results and discussion

Treatment of solid state data

The original aim of this work was to study films obtained from chiral porphyrin J-aggregates via VCD-IR spectroscopy. First, we illustrate how we will analyze such data. Solid state crystalline and film analysis by VCD was first carried out by Buffeteau et al.,²⁶ Merten et al.,²⁷ Lombardi and Nafie,^35,61 Polavarapu¹⁴ (as recently reviewed³¹ by Krupová and Andrushchenko). These studies build upon the seminal work by Shindo and Nakagawa,³⁴ who utilized Mueller matrix analysis^62–64 mainly to investigate CD in the UV-vis range. This type of analysis allows to evaluate the linear contribution effects of the solid state within the observed CD spectral data (CD_obs), corresponding namely to the actual signal recorded by CD instrumentation. This means that the recorded spectra may be sensitive to rotation or flipping of the solid-state sample with respect to the direction of the light beam. In the following equation are indicated the various most important factors contributing to CD_obs in the solid state:^26,27where CD is the “true CD” (position-insensitive to sample rotation and flipping) and the second term relates to linear dichroism (LD/LD′) and linear birefringence (LB/LB′) taking place at different axes rotated by 45°, which will be here reported as LDLB. Terms averaging out upon rotation around the beam direction and higher order terms as discussed in ref. 65, are left out of eqn (1). A couple of comments are in order. First, as stated by Shindo³⁷ in a paper collecting his patient and constant work through the years and by Troxell and Scheraga et al.,³² eqn (1) derives from a power series development of the Mueller matrix, with the first term being linear in film thickness and the second one being quadratic in film thickness. Parenthetically we wish to report that, in his work, Shindo recognized the great merit of Gō³⁸ in explaining and reviewing the Mueller matrix method. Secondly, one should include more contributions to eqn (1), as generated by instrumental artifacts, including, among others, static birefringence of photo-elastic modulator (PEM) and linear contributions that come from monochromators, photomultipliers, source, linear polarizer, etc.; such contributions generate spurious signals when the light interacts with samples showing LD and/or LB and can be partially compensated by rotating the solid sample along the light beam axis direction.^34,61

The first and the second terms of (1) can be extracted as the semi-sum and semi-difference of front and rear obtained VCD observed spectra. Front and rear position are considered as reported in Scheme 2: we repeat here that front means that the film faces the incident light beam and rear the opposite position.^27,66 Starting from eqn (1) and according to considerations reported in ref. 27, it is possible to define the following quantities in eqn (2):

Scheme 2 Schematic representation and definition of the VCD/IR experiments on J-aggregates TPPS₄ films. Green layers depict the film layer and CaF₂ plate (in white) relative position.

The previous analysis has received increasing attention in the recent literature: on one side a clearer rationalization has been reached, secondly applications are possible of what originally were thought artifacts to avoid. Starting from the study of chiroptical effects of plasmonic nanostructures⁶⁷ the concept of “2D chirality” has been introduced and adopted also for CD in the solid state. A 2D-chiral object presents two planar structures that cannot be brought one into the other whichever symmetry operation is performed within the plane. The CD inversion behavior due to “2D chirality” can be exploited in a Fabry–Pérot cavity to enhance the chiroptical signal.⁶⁸ By analyzing various examples of films presenting prevalent LDLB component, the presence of anisotropic birefringent domains has been evidenced.^69,70 Another component could be circular differential scattering associated to crystallization domains.⁷¹

VCD of tartaric acid in different preparations

Since the main aim of the present study was to characterize with VCD chiral porphyrin aggregates templated by tartaric acid used in large molarity ratio with respect to TPPS₄ (see in the following) we preliminarily examined VCD spectra of tartaric acid (TA) per se, measured in different conditions, that is in DMSO solution and different solid-state preparations. As expected, data taken at solid state reveal an enhancement of VCD signals but also an important LDLB contribution to the overall spectra. We report in Fig. S3 VCD-IR spectra of TA in deuterated DMSO solution (data compare well with ref. 72) and as KBr pellets. We would like to recall that the first VCD spectrum on TA was recorded by Sugeta et al.⁷³ in the CH stretching region, and recently Sato et al. measured VCD of TA in an organoclay nanohybrid with quantum cascade laser (QCL) setup.⁷⁴ In the past, IR spectra of KBr pellets as well as of films and melt tartaric acids had been studied, in optically active form, in meso-form and as racemates. The major differences were found in the OH-stretching region, while mid-IR seems less sensitive to the different preparation procedures.^73,75–77 Commenting VCD data (Fig. S3), we may notice a ca. three times intensification of the experimental g dissymmetry ratio from solution to KBr pellets. It is also important to report that KBr pellets did not provide any anisotropic effect and, consequently, no change of VCD spectra upon flipping or rotating pellets was observed. This is because of the efficacious grinding process of the solid compounds, thus ensuring homogeneous dispersion of the sample in the pellet.¹⁹

The large dissymmetry ratio represents an advantage in employing solid state measurements since shorter accumulation time is required with respect to solution measurements: on our apparatus, each solution measurement generally needs 6000 accumulations (about 1 hour) while KBr pellet measurements needed only 2000 accumulations to provide clear VCD spectra with acceptable signal to noise ratio.

We then drop-cast a tartaric acid film on CaF₂ plates starting from a 100 mM H₂O solution. TA does not give rise to homogeneous films on the plates but it rather crystallizes in spots providing very clear and intense anisotropic “front/rear” features in observed VCD spectra which are one order of magnitude more intense than “true CD” contributions (Fig. S4). The fact that front and rear spectra of each enantiomer are nearly mirror image implies that the main contribution is from LDLB term: applying eqn (2) one obtains LDLB spectra with dissymmetry factor g ca. 10⁻¹ (10⁻³ for CD). It should be noted that in these latter conditions the measurements required only 50 accumulations (about 2 minutes) to provide reliable spectra. IR spectra (used together with VCD to evaluate g-factors) are independent on both the orientation of the film and also upon flipping from front to rear and, and they are nearly identical for the two enantiomers of TA.

Preparation of TPPS₄/TA films and VCD analysis

After checking the high variability of TA VCD spectra in different physical states of the sample, we examined cast films obtained from TPPS₄/TA aggregates. In order to create the proper chiral environment which is able to enantiomerically drive TPPS₄ J-aggregation, 100 mM solutions of D/L-TA were added to a 1 mM porphyrin solution prepared according to a “porphyrin first” protocol.⁴¹ In the first series of experiment we mixed the two solutions 6 : 4, i.e. 6 parts in volume of TPPS₄ solution to 4 parts of TA solution. Starting solutions were prepared using both H₂O and D₂O as solvent and we monitored the formation of both aggregates via ECD-UV spectroscopy (J-aggregates driven by D-TA and L-TA will be defined here as J_D and J_L respectively). Seven days old J_D and J_L solutions showed enantiomeric ECD couplets as reported in Fig. S2. As one can note, J-aggregation took place to a good degree, showing intense enantiomeric couplets centered at 490 nm while there is a decrease of the 434 nm UV band, this latter being allied to the TPPS₄ diprotonated monomeric form. The H aggregate component is difficult to identify within the absorption spectrum, since it appears as a shoulder only slightly blue-shifted with respect to the monomeric absorption band: however, the CD doublet centered at 420 nm can be considered as indicative of the presence of H-aggregate component. The two J and H aggregation mechanisms are present within the TPPS₄ nanotube as described in reference⁷⁸ Furthermore, we can notice an isotopic effect:⁶⁰ D₂O gave one order of magnitude more intense ECD with respect to ordinary water (see also the UV spectra) after seven days. In D₂O solutions, ECD bands related to Q bands are also detectable in the spectrum (Fig. S2, inset) not present in the H₂O aggregates. This isotopic effect was already noted⁶⁰ and spectroscopically characterized for TPPS₄ aggregates formed in very diluted acidic solutions (about 3 μM) comparing DCl/D₂O and HCl/H₂O mixtures, identifying inverse kinetic and equilibrium isotope effects. In general, it is often the case that, due to the lower vibrational frequency and thus due to a lower zero point vibrational energy,⁷⁹ deuterium gives energetically more stable aggregates and has influence on the kinetics promoting a faster process and so ultimately controls nanostructure formation.⁸⁰

For the VCD experiments, J_D and J_L drop-cast films on CaF₂ plates were prepared from the previously described solutions, after checking with ECD the proper aggregate formation, and both H₂O and D₂O preparations were used. 24 hours were enough to obtain films at 20 °C.

The VCD-IR spectra of the films actually showed only TA signals (due to larger amounts of TA with respect to TPPS₄ in the parent solutions).

All VCD spectra taken on TPPS₄/TA films are practically insensitive to plate rotation around the radiation beam (Fig. S5). LDLB contribution is large, as immediately suggested by the fact that the front and rear positions yield quite different recorded spectra (Fig. 1), though not with all features of opposite sign as those observed in pure TA films; in that case, the CD contribution was only a minimal part of the recorded signal.

Fig. 1 VCD spectra comparison of front and rear (dark/light color traces respectively) position of films of J_D (upper panels) and J_L (lower panels) aggregates. Reported VCD spectra refer to H₂O (left panels) and D₂O (right panels) drop cast solutions on CaF₂ plates (see text). Experimental conditions: [TPPS₄] = 0.6 mM; [TA] = 40 mM (lower panel). Molar ratio [TA/TPPS₄] ≈ 67.

Taking into account eqn (2), one can extract from the recorded spectra of Fig. 1 the CD and the LDLB contributions which we report in Fig. 2. It is worth noting that CD and LDLB spectra are characterized by absolute intensities of similar order of magnitude, which is indicative of the fact that the presence of porphyrin aggregates is more efficient in providing reliable CD data than simple TA drop-casting; in the latter case in fact anisotropic effects were at least one order of magnitude more intense than “true” CD (Fig. S4). One may also notice that both H₂O and D₂O films show nearly identical absorption spectra in the recorded spectroscopic region (see also ref. 77) (Fig. S6), and quite similar, albeit not identical, VCD spectra, (Fig. 2, upper panels), while LDLB spectra are more substantially different (Fig. 2, lower panels); this may suggest that the LDLB component is strictly tied to aggregate formation kinetics and their film deposition, while CD is less sensitive and more reproducible considering different preparation following the same protocol.

Fig. 2 Comparison of J_D (blue lines) and J_L (red lines) aggregate VCD spectra obtained by summing the recorded spectra of Fig. 1 (indicated as CD, upper panels) and of LDLB anisotropic contribution obtained by subtracting the data of Fig. 1 (lower panels). VCD spectra in the left panels refer to H₂O and in the right panels to (D₂O drop cast solutions on CaF₂ plates). Experimental conditions: [TPPS₄] = 0.6 mM; [TA] = 40 mM (lower panel). Molar ratio [TA/TPPS₄] ≈ 67.

Varying LDLB contribution

The obtained similarity of the VCD spectra for these films make us confident to consider such templation mechanism useful in handling anisotropic effects of the solid-state conditions in the recorded spectra. Such an approach was impossible to apply for the cast film of mere TA, where true CD contribution was too weak with respect to LDLB. This indeed means that by treating TA-porphyrin aggregates we got some control over the drop-casting procedure which is for sure straight, fairly quick and simple but not always exactly reproducible.⁸¹ Once established the efficiency of TPPS₄ in promoting formation of regular films to obtain solid state VCD spectra, we changed TPPS₄ : TA mixture ratio from 6 : 4 to 8 : 2, i.e. we performed drop-casting of J_D and J_L aggregates by mixing 800 μL of TPPS₄ and 200 μL tartaric acid of previously described stock solutions (see Fig. S7), in this case the final concentration of acid is 25 times that of porphyrin. In Fig. S8 and S9 we report the comparison of true CD and LDLB VCD spectra, respectively, for both 6 : 4 and 8 : 2 J_D and J_L mixtures. Interestingly, one may notice that both VCD spectra and LDLB contributions, in the case of 8 : 2 films, appear to be more regular and enantiomeric with respect to the previous 6 : 4 case.

Coming now to an analysis of VCD spectra, we observe that the two sets of films show very similar features in the range 1100–1500 cm⁻¹ while, in the lower wavenumber region (850–1000 cm⁻¹), the triplets exhibited in 6 : 4 films (Fig. S8, upper panels) are not detected in the 8 : 2 ones. The 1650–1850 cm⁻¹ region with CO stretching features shows an overall similar trend, while relative intensities change when changing TA content or solvent (H₂O versus D₂O). Analyzing the origin of these differences is not easy since the exact structures/conformations of the molecules within the film are not known. VCD is highly sensitive to molecular conformations which can be stabilized by differences in molecular packing that in turn depend on the TPPS₄ content, on the kinetics of aggregate formation in solution, and on the casting process.

As previously reported, acidic conditions promote J-aggregation.⁴³ For this reason we decided also to drop-cast films obtained from 8 : 2 (TPPS₄ : TA) J_D and J_L water solutions with further addition of HCl (20 μL 1 M HCl solution in 500 μL 8 : 2 J-aggregate solutions) in order to investigate whether the VCD spectra are affected by such conditions. The acidification effect was monitored by ECD-UV in which the increase of the aggregate 490 nm band with respect to the 434 nm of the monomeric form is quite evident (see Fig. S10). In this case, we still confirmed rotation-insensitive VCD spectra of J_D and J_L films but also absence of LDLB contributions. VCD spectra of front and rear orientations for both J_D and J_L samples are almost superimposable (see Fig. S11 upper panels) and comparison between data of films obtained from aggregates in HCl-acidified solutions and data of original 8 : 2 films shows very similar features in the whole 850–1850 cm⁻¹ region (Fig. 3). J_D and J_L data appear almost enantiomeric confirming the ability of TPPS₄ to promote regular film templation, furthermore, this result suggests that the more the J-aggregation process is completed in solution the less the films are affected by anisotropic contribution. Since J-aggregates of TPPS₄ are formed by linear arrangements of porphyrins, often leading to well-structured nanotubes, the experimental evidence could be ascribed to the ability of such nanostructures of interacting and in some way controlling the TA environment during the formation of the films. This effect should be stronger in the presence of longer nanotubes or fibers. In fact, passing from less to more acidic conditions by addition of inorganic strong acid, J-aggregates are expected to grow longer as proved by the increase of the 490 nm band in the UV-Vis spectra.

Fig. 3 Comparison between J_D (blue lines) and J_L (red lines) aggregate VCD spectra as obtained by summing the originally recorded spectra on drop cast film from H₂O/HCl solution: [TPPS₄] = 0.77 mM; [TA] = 19.2 mM; [HCl] = 38.5 mM (upper panel) and H₂O solution: [TPPS₄] = 0.8 mM; [TA] = 20 mM (lower panel) (Molar ratio [TA/TPPS₄] ≈ 25).

Details about the structural aspects of the prepared films accounting for the different spectroscopic response deserve further future investigations, herein we just try to clarify the role played by TA, TPPS₄ and acid addition in the resulting spectroscopic behavior.

The highest LDLB content is undoubtedly observed for pure TA films and the reason for that and for the macroscopic inhomogeneity of the films is allied to its propensity to crystallize. TA acts as the chiral seed, initiating and guiding the emergence of chirality at increasing hierarchical levels, promoting chiral manifestations of all preparations. TA crystals are 3D-chiral superstructures: when observed in pellets, that is in isotropic medium, they give “true” CD (unless there is some preferred orientation induced during the preparation of the pellet, but that does not seem to be the case within the sensitivity limits of our instrumentation). When TA crystals grow on a window plate, they have an up and down direction and are no more isotropically distributed. Having an above-sided and a below-sided face, the resulting CD spectra are different when light impinges on either sides giving a very similar situation to what has been called in the recent literature “2D-chirality”.^67,69,70 From Fig. S13a and S14a, one can observe that TA crystals organize in elongated structures covered by semi-amorphous material.

TPPS₄ nanotubes in solutions are 3D chiral superstructures with chirality induced by TA, and they are formed in an isotropic environment and are randomly oriented. As the solution dries, TPPS₄ aggregates impede or, better, limit and continue the extended formation of TA crystallites, that appear as isolated entities (see Fig. S13b, c and S14b, c); concurrently, TPPS₄ fibrils’ spatial arrangement is influenced by TA crystallization nuclei. So, while the TPPS₄ nanotube component is organized in fibrils (see Fig. S13b, right panel) and is perturbed by TA crystallites, thus exhibiting high variability in its ECD response in the UV-VIS region, on the contrary, the same films yield highly reproducible results when analyzed by VCD spectroscopy, which selectively probes the randomly distributed TA crystallites: LDLB spectral contribution, in fact, is indicative of a residual crystallization. In any case, “true” VCD can be clearly disentangled from the front and rear recorded spectra, the lower the TA:TPPS₄ ratio the better (see Fig. S7).

The results obtained upon addition of HCl to the TPPS₄/TA water solution confirm the above description, assuming that HCl modifies TA crystallization: in this last case, in fact, not only VCD spectra are invariant upon flipping (no detectable LDLB component), but also UV-VIS ECD spectra, where TPPS₄ absorbs, are reproducible and do not show sensible changes upon flipping (Fig. S11 and S12). From Fig. S13d and S14d one can see that samples rearrange in a different fashion: crystals are still present but showing a microcrystalline dendritic or fractal structure.

All the above considerations indicate that TA crystallization nuclei are responsible of the LDLB component at VCD and also of perturbing the TPPS₄ nanotube arrangement.

Conclusions

In this work, we have made use of diacid TPPS₄, which is easily driven to form chiral J-aggregates whose handedness is determined by templation with either D or L enantiomers of TA: aggregation in H₂O and D₂O solutions can be easily monitored by ECD-UV technique. Once aggregation in solution is established, films are deposited by drop-casting on CaF₂ plates. The VCD-IR technique confirms that the combination TPPS₄/TA promotes the formation of uniform, reproducible films leading to consistent optical properties across multiple samples. It is also shown that chiroptical properties of such films can be tuned upon changing the relative molar ratio of the porphyrin system and of TA or upon acidification and even by considering D₂O instead of H₂O starting solutions. While “true” VCD main features (invariant upon flipping the depositing CaF₂ plate from front to rear orientation) are common to different film preparations, linear dichroism-linear birefringence components (LDLB term) seem strictly connected to aggregate formation kinetics and to film deposition. All film measurements showed a dissymmetry factor of 10⁻², which is quite high with respect to common dissymmetry factor registered in the IR range.

The fact that the TPPS₄ peculiar properties permit to strongly influence and direct the aggregation and subsequent film formation process such as to have some control over the resulting drop-cast films, is indeed important in view of the growing interest towards optically active films employed as optoelectronic layers or for tuning absorption/emission of circularly polarized light or for sensing and many other applications. In this context VCD spectroscopy reveals to be an important tool to characterize aggregation states in films at multiple hierarchical levels, testing the quality of chiral supramolecular solid-state arrangement.

Regarding the two actors of this study, this investigation provides useful addition to the studies of TA in the solid state, which have a long history in chirality, ranging from the seminal separation of TA enantiomers via crystallization à-la-Pasteur⁸² to one of the first determination via X-ray diffraction⁸³ to neutron-scattering studies⁸⁴ and recent co-crystallization studies.^85–87 Regarding porphyrin and chiral supramolecular structures thereof, we recall their recent use for basic studies of phenomena like MCD and magnetochiral dichroism (MChD),^54,55 for the development of good substrates to evidence Chiral Induced Spin Selectivity (CISS) effect^88,89 and for application as sensor of chiral excess in inorganics⁹⁰ or vapor enantiomers recognition⁹¹ just to cite some examples.

Materials and methods

Tetrakis-(4-sulfonatophenyl)porphyrin (TPPS₄) and tartaric acid (TA, D and L) solutions were obtained by dissolving the solids in H₂O and D₂O to 1 mM and 100 mM concentrations respectively. J_D and J_L aggregates were obtained by adding TA solutions in the TPPS₄ porphyrin solution in either 4 : 6 or 2 : 8 ratio. The further acidification procedure consisted in adding 20 μL of a 1 M HCl solution to 500 μL J-aggregates mixtures.

J_D and J_L films were obtained by drop cast depositing 60 μL of TA/TPPS₄ mixtures on 13 mm-diameter CaF₂ plates and letting the solvent to evaporate for 12 hours.

VCD/IR measurements were carried out using a Jasco FVS 6000 apparatus equipped with a ZnSe photo-elastic modulator (PEM), working at 50 kHz frequency modulation, placed past a wire grid linear polarizer and synchronously gauged to a lock-in amplifier. An MCT liquid-N₂ cooled diode-detector for the regions 850–2000 cm⁻¹ was employed. VCD spectra of films were recorded by rotating the sample in 90°-steps around the light beam axis and by also flipping the samples. All solid state VCD spectra are reported as average over the spectra of the four rotated films. 6000 scans were acquired for DMSO-d₆ measurement and solvent spectra were taken in the same conditions and subtracted out. 2000 scans were acquired for KBr pellets TA and 50 accumulations for films measurements. Resolution was set at 4 cm⁻¹. ECD/UV measurements were conducted with Jasco 815SE with 0.1 mm quartz cell in H₂O and D₂O solutions. ECD/UV measurements have also been conducted on the same film deposited on CaF₂ plates.

For microscopy investigations 20 μL of the solutions containing TA, or the TA/TPPS₄ mixtures were deposited by drop cast onto commercial 4-inch silicon wafer 100-orientation, 500 μm thickness, n-type 1–5 Ohm cm⁻¹ of resistivity (Siegert Wafer). Scanning Electron Microscopy (SEM) images were acquired by using a Zeiss Gemini 460 by using an InLENS detector and 10 kV of extraction voltage.

Digital Microscope images were obtained by using a Keyence VHX7000 in backscattering and dark field conditions.

Author contributions

All authors have contributed to the final version of the manuscript.

Conflicts of interest

There are no conflicts to declare.

Data availability

All experimental procedures and characterization data are available in the supplementary information (SI). Supplementary information: ECD spectra of TPPS₄ + tartaric acid systems in H₂O and D₂O solutions. IR and VCD spectra of tartaric acid enantiomers in KBr pellets and DMSO solution and as films prepared by evaporating the solvent. VCD spectra of TPPS₄ + tartaric acid systems prepared from H₂O and D₂O solutions in various orientation and conditions. ECD spectra of TPPS₄ + tartaric acid systems with addition of a small HCl quantity. SEM and optical images of the TA and TPPS₄ + tartaric acid systems. See DOI: https://doi.org/10.1039/d5cp03069b.

Acknowledgements

Funding by the Italian Ministry of University and Research (MUR): projects SMART HELIX, prot. 2022B3EFJH, INVESTCPE, prot. 2022CXHY3A, CHI-EP (PRIN_2022HPW79T_002) and Next Generation EU, PNRR Samothrace Project (ECS00000022) is acknowledged. The authors acknowledges also the Italian Project PNNR “I-PHOQS—Integrated Infrastructure Initiative in Photonic and Quantum Science” CUP B53C22001750006 and Prof. Antonio Leonardi for SEM and optical microscopy. The authors wish to thank the reviewers, who provided insightful suggestions for improving the manuscript and for pointing out relevant literature.

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