A Feasible Single-Solvent Nucleation and Growth Protocol for the Well-Defined Organization of Simple Porphyrins on Different Glass Composites

Abstract

Herein we report the nucleation and growth of porphyrin molecular chromophores using a single-solvent deposition protocol. Various glass substrates were investigated, aiming to investigate their impact on the organization of tetraphenyl-porphyrin (TPP) towards well-defined architectures. A variety of aggregation morphologies were obtained upon optimizing several parameters, including the solvent and the temperature of evaporation. This work demonstrates for the first time that single-solvent evaporation results in nanostructures, avoiding the necessity of mixed-solvent reprecipitation. Additionally, we showed that simple symmetrical porphyrins do not need the presence of self-assembling peptides, ions or amphiphiles to induce the capability of forming well-defined structures. The results presented herein open new avenues for the development of complex and highly ordered architectures from simple building blocks towards advanced materials with tailored properties.

1. Introduction

The self-assembly of small molecules into well-defined architectures without external direction is an interesting and widely investigated process in the fields of nanotechnology and materials science [1]. The most common approach involves the utilization of peptides [2], ions [3] or amphiphiles [4] as self-assembling inducers, which can form highly ordered structures through non-covalent interactions such as hydrogen bonding, coordination bonding, π-π stacking, van der Waals forces and electrostatic and hydrophobic interactions [5,6]. The resulting self-assembled nanostructures include micelles, spheres, vesicles, fibrils and nanotubes, which have been applied in various applications [7,8,9].

Porphyrins, a class of tetra-pyrrole macrocyclic chromophores, have gathered significant attention due to their exceptional photophysical and electrochemical properties, rendering them promising candidates in catalysis [10], sensing [11], photovoltaics [12], drug delivery [13], tumor imaging [14] and photodynamic therapy [15]. The self-assembly of porphyrins is particularly important because it allows for precise control over their spatial arrangement, which directly influences their optical, electronic and photophysical properties [16,17]. When porphyrins self-assemble into ordered arrays, they exhibit increased light-harvesting ability, improved charge transfer phenomena and tunable fluorescence emissions [18]. These organized structures are essential for applications like gas sensing, photovoltaics and catalysis, where the performance of the material is critically dependent on the arrangement and interactions of the chromophores [16,17]. Therefore, understanding and controlling the solid-state nanostructures of porphyrins is key to unlocking their full potential in cutting-edge technologies [19].

The well-organized deposition of molecular chromophores on solid substrates, particularly glass substrates, offers several benefits that improve both the material properties and the potential applications of these systems [20]. The organized arrangement of chromophores on these substrates can lead to enhanced optical phenomena [21], which are particularly beneficial in devices such as sensors, light-emitting diodes (LEDs) [22] and photovoltaic cells [23]. Inorganic oxide glass substrates also provide a chemically inert and physically stable environment for the chromophores in order to maintain their structural integrity over time, improve their optical properties and enhance the stability and durability of the so-formed device [24]. Among inorganic oxide glasses, phosphate-based glasses offer a feasibly tunable surface chemistry [21,25], which can be tailored to induce or control the crystallization nucleation and growth of molecular chromophores in terms of orientation and density. This tunability is crucial for optimizing performance in applications such as waveguides, photonic devices, bio-imaging or sensors. Finaly, phosphate glasses can be doped with metal nanoparticles (NPs) in order to introduce advanced plasmonic features [26] that, when combined with well-organized chromophores, can facilitate efficient charge separation and transfer. This is particularly advantageous in dye-sensitized solar cells [27], where effective charge transfer is essential for improved power conversion efficiencies [28].

Despite the above-described advantages, achieving a well-defined, controllable and reproducible organization of porphyrins on solid surfaces remains a challenging task [29], especially when aiming for simplicity in the assembly process. Traditionally, porphyrin self-assembly on substrates has been achieved using a combination of solvents (“good” and “poor” solvents for reprecipitation), often requiring multiple steps and multistep chemical reactions to connect self-assembling inducers on the periphery of the macrocycle [30]. The reprecipitation protocol involves a “good” solvent where the building blocks are molecularly dissolved, so they are in their monomeric form, while the introduction of a “poor” solvent induces aggregation [16]. These protocols frequently involve the co-assembly of other molecules or the assistance of surfactant agents to guide the organization of porphyrins, adding extra complexity, which limits scalability [31,32]. Based on the above, the need for simpler and more direct methods to achieve precise molecular organization has driven us to propose a rather simple single-solvent approach.

In this study, we introduce a novel single-solvent nucleation and growth protocol for the well-defined organization of simple porphyrins on three distinct glass substrates: silver phosphate (AgPO₃), sodium phosphate (NaPO₃) and typical silicate (SiO₂). By optimizing the solvent and deposition conditions, we have developed a method that facilitates the formation of highly ordered porphyrin assemblies with minimal processing steps. Importantly, our approach does not rely on the use of surface modifiers or additional self-assembling inducers like peptides, ions or amphiphiles, rendering it a straightforward and potentially scalable technique. The proposed protocol was demonstrated on AgPO₃, NaPO₃ and SiO₂ glass substrates, each offering unique surface characteristics that affect the interaction with the porphyrin molecules. Through a combination of microscopic and spectroscopic analyses, we have examined the resulting porphyrin films, elucidating the key parameters that govern the formation of these organized structures. The results demonstrate that this feasible single-solvent approach is broadly applicable across different substrate types, providing a versatile tool for the fabrication of porphyrin-based materials. The implications of this method extend beyond the specific systems studied, providing insights that could be applied to other molecular assemblies and potentially leading to new applications in material science and nanotechnology.

2. Experimental

2.1. Materials Synthesis

All reagents were purchased from common commercial sources and used without any further purification, unless otherwise stated. In brief, silver metaphosphate glass (AgPO₃) and sodium metaphosphate glass (NaPO₃) were synthesized using AgNO₃ (99.995%), NH₄H₂PO₄ (99.999%) and Na₂CO₃ (≥99.9% dry basis) powders as starting materials, following known experimental procedures [21,33]. 5,10,15,20-Tetraphenyl-21H,23H-porphine (TPP) was synthesized following the Alder–Longo method [34]. The SiO₂-based glass substrate (typical microscope cover glass) was purchased and cleaned thoroughly via a standard cleaning protocol: sonication in soapy water (15 min), then water and finally ethanol. After rinsing with ethanol, the SiO₂-based glass substrates were dried in ambient air.

2.2. Characterization

UV–Vis absorption studies were conducted using a Shimadzu UV-1700 spectrometer (Shimadzu Europa GmbH, Germany). Emission spectra were recorded using a JASCO FP-6500 fluorescence spectrophotometer (Jasco Int. Co. Ltd., Tokyo, Japan) equipped with a red-sensitive WRE-343 photomultiplier tube (wavelength range: 200–850 nm). Both UV–Vis and emission spectra were obtained in solid state using the porphyrin/glass sample as obtained after the drop-casting. Scanning electron microscopy (SEM) images were acquired using a field-emission scanning electron microscope (JEOL, JSM-7000F) (JEOL Ltd., Tokyo, Japan) equipped with an INCA PentaFET-x3 EDS detector. Raman spectra were collected using a Nicolet Almega XR Micro Raman analysis system (Thermo Fisher Scientific, Waltham, MA, USA) equipped with a diode-pumped solid-state (DPSS) laser. The excitation wavelength was set to 473 nm.

2.3. Abbreviations

TPP: 5,10,15,20-Tetraphenyl-21H,23H-porphine.

DCM: dichloromethane.

THF: tetrahydrofuran.

3. Results and Discussion

3.1. Microscopy Investigation

After the successful preparation of NaPO₃ and AgPO₃ transparent glass substrates, we proceeded with the investigation of the crystal nucleation and growth of the 5,10,15,20-Tetraphenyl-21H,23H-porphine (TPP) chromophore. Towards this target, we followed a rather simple protocol based on the solvent evaporation, via drop-casting molecules dissolved in a chaotropic solvent, on different glass substrates and at different temperatures. A schematic representation of the single-solvent nucleation and growth protocol is illustrated in Scheme 1. Initially, TPP was dissolved in a “good” or chaotropic solvent at a concentration of 1 mg/mL and then it was drop-casted on a preheated glass substrate. Several drops of the TPP solution were added, with a 20-s interval time between each addition to ensure the gentle vaporization of the corresponding solvent and the formation of a solid film. As a “good” solvent, where porphyrin molecules are in their monomeric form, we tested dichloromethane (DCM) and tetrahydrofuran (THF), while the various temperatures included room temperature, a temperature close to the corresponding boiling point of the solvent and a considerably higher temperature (namely, 170 °C).

Scanning electron microscopy (SEM) images were obtained to establish the aggregation mode of the porphyrin molecules on the different glass substrates. Figure 1 depicts the SEM images of TPP drop-casted at 25 °C on NaPO₃, AgPO₃ and SiO₂ from an identical THF solution. Namely, we observed rectangular supramolecular architectures in the case of NaPO₃ and flower-like nanostructures in the case of SiO₂ glass, while AgPO₃ did not show any organized structures. This result clearly demonstrates that the nature of the glass substrate affects the morphology. In the case of TPP, the NaPO₃ substrate was found to be the most promising towards well-shaped nanostructures, followed by SiO₂ glass. These observations can be explained by variations in the surface roughness, in addition to the interactions between the molecular chromophore and the glass network. Such interactions involve ionic interactions, hydrophobic interactions, hydrogen bonding and van der Waals and coordination linkage [35,36,37]. The outermost surface is expected to be mostly oxygen-terminated in all substrates, but AgPO₃ and NaPO₃ glass have phosphonate moieties (PO₄³⁻), which will be upsent in the SiO₂ substate [37]. Noteworthily, the major differences in the AgPO₃ and NaPO₃ substrates are the presence of Ag nanoparticles (Ag⁰) in the first case and the soft nature of Ag⁺ ions [38]. These factors might cause disorder (or order), limiting (or facilitating) further crystal nucleation and growth [39].

Moving one step forward, we examined the influence of various temperatures by studying the resulting morphology of TPP upon evaporation from THF on the NaPO₃ glass substrate at three different temperatures (namely, at 25 °C, 70 °C and 170 °C). As shown in Figure 2, by increasing the temperature close to the boiling point of the THF solvent, the supramolecular architectures were improved in terms of shape, while the rapid evaporation at a considerably high temperature (170 °C) inhibits the nucleation and growth process. Noteworthily, the homogenous and aligned rectangular sticks of Figure 2b obtained at 70 °C have a higher length compared to those in Figure 2a obtained at room temperature. This observation proves that the temperature affects the resulting morphology and is consistent with previous reports [40,41]. This result could be attributed to the higher amount of energy provided to the system, assisting the crystal nucleation and growth process, when the substrate is heated to the boiling point of the solvent, whereas at really high temperatures the system does not have enough time to assemble into a distinctive mode (Figure 2c) [39].

The last parameter investigated herein was the effect of the solvent on the nucleation and growth process. As expected, the “good” solvent used in the proposed single-solvent nucleation and growth protocol significantly affects the morphology of the nanostructures. This result is derived from Figure 3, which depicts the nanostructures obtained when DCM, instead of THF, was used as a “good” solvent for drop-casting TPP on NaPO₃ and SiO₂ at 25 °C and 40 °C. This solvent generated both sticks and tubes, depending on the temperature and the glass substrate. Comparisons between Figure 3a and Figure 2a or Figure 3b and Figure 1c clearly show the impact of the evaporation solvent, since when the same substrate and same temperature are used different assemblies are obtained. Indeed, well-defined architectures were obtained in the absence of protic solvents or hydrogen-bonding entities. Based on this, we conclude that the primary reason for the nucleation and growth ability of these systems is the π-π stacking, as well as the van der Waals interactions involving the porphyrin moiety [42].

In an effort to further support this finding, we investigated the influence of two additional substrates with a significantly different surface tension compared to NaPO₃, namely Si substrate and FTO-coated glass [43,44,45,46]. As depicted in Figure S1, the obtained nanostructures (sticks and tubes) exhibited a similar morphology to those presented in Figure 3. The only difference was the substrate on which the templated nucleation and growth process initiated. Since the substrates utilized in this work have different surface tensions ranging between 0.1 and 1.2 J/m² [45,47], we conclude that this factor plays a less significant role in determining the resulting morphology, compared to the solvent and the temperature during drop-casting. Additionally, we present the statistical size distribution of the rectangular sticks obtained in three different cases through the histogram in Figure S2.

3.2. Spectroscopic Investigation

The nucleation and growth of porphyrin chromophores to distinctive architectures is well known to alter the optical properties of these nanostructures. Optical absorption and emission spectra of the prepared samples with deposited TPP on different glass substrates were studied, in order to provide further insights and elucidate the formation of “H” or “J” aggregates [47,48]. In general, porphyrins display distinct sharp absorption bands in the visible region, while appropriate stacking or aggregation can broaden or shift the light absorption range. UV–Vis absorption spectra of the monomeric TPP (in solution), along with the corresponding fluorescence emission spectrum, are presented in Figure S3.

In detail, TPP shows a strong Soret band maxima at 418 nm and four weak Q-bands at 514, 549, 590 and 646 nm, while the emission peaks after excitation at the Soret band are found at 652 and 715 nm. After the nucleation and growth of TPP molecules into well-defined nanostructures, the Soret band becomes significantly broadened, red-shifted, and in the case of the rectangular architectures on NaPO₃, it splits into shoulder and prominent peak maximum at 420 nm (Figures S4 and S5). Noteworthily, at room temperature evaporation of THF solvent, the substrate which illustrated the highest redshift was AgPO₃, which showed no nanostructures. Red-shifting was also observed in the fluorescence spectra, along with the appearance of a third peak at 755 nm (Figures S4b and S5b). The above differences in the electronic absorption spectra of the assemblies clearly reveal alterations in molecular stacking and aggregation, which are also seen in the SEM observations.

While investigating the influence of the evaporation temperature, which is directly corelated to the evaporation rate, we found an interesting motif in the UV–Vis and emission spectra. As presented in Figure 4a, by increasing the solvent evaporation rate on NaPO₃, the Soret band is further red-shifted, and again we observed that the absence of well-organized nanostructures resulted in the highest redshift. Interestingly, the ratio ${\displaystyle \frac{lastQbandintensity}{Soretbandintensity}}$ in Figure 4a increases by increasing the evaporation temperature, along with the intensity of the newly appeared emission peak in Figure 4b, which follows the same pattern. These two factors could provide a quantitative spectroscopic tool to “measure” the organization in these systems.

From the above absorption and emission spectra analysis, we can conclude that the nanostructures prepared herein consist of J-aggregates (side by side), since red-shifted peaks were obtained in all cases (Figure 4, Figures S4 and S5). Table S1 provides a detailed list of all peak maxima for both absorption and emission spectra. Variations in the shifting of peaks and alterations in the peak intensities are observed due to different aggregation and stacking phenomena, as observed in the corresponding SEM studies.

Finally, Raman spectroscopy was employed to probe any structural modification of the porphyrin molecules upon the described nucleation and growth process. In particular, Figure S6 presents the room temperature Raman spectrum of TPP on the three different glass substrates. As expected, no significant alterations were observed, since nucleation and growth is an intermolecular process and does not influence the chemical bonding and structure within the porphyrin molecules nor the glass-attributed Raman features. Namely, the obtained Raman features are attributed to the wagging of the phenyl Hs at ca. 825 cm⁻¹, the C_m-C_p bond stretching at 1234 cm⁻¹ and the asymmetric stretching and bending deformation of C_a-C_m-C_a at ca. 1550 cm⁻¹ [49].

4. Conclusions

In summary we have presented a novel, simple and straight forward nucleation and growth protocol for symmetrical porphyrin chromophores on glass substrates, based on feasible drop-casting from a single-solvent system. We examined the influence of the glass substrate on the organization of tetraphenyl-porphyrin towards well-defined architectures. More specifically, we employed AgPO₃, NaPO₃ and SiO₂ glass surfaces, and we obtained a variety of aggregation morphologies upon altering the employed solvent and evaporation temperature. The present study demonstrates for the first time that single-solvent evaporation generates well-defined nanostructures. Consequently, the necessity for mixed-solvent systems and the presence of peptides, ions or amphiphiles as self-assembling inducers is avoided. The observed rectangular supramolecular architectures, the flower-like nanostructures, the sticks and the tubes were all obtained from the same chromophore molecule just by slightly altering its environment, i.e., the substrate and solvent. The absorption and emission spectra analysis showed that these nanostructures consist of J-aggregates (side by side), since red-shifted peaks were obtained in all cases. The results of this work could open new avenues for the development of complex and highly ordered architectures from simple building blocks towards advanced materials with improved properties.

Author Contributions

Methodology, E.N. and I.K.; Formal analysis, E.N.; Investigation, E.N.; Data curation, E.N.; Writing—original draft, E.N.; Writing—review & editing, I.K. and E.S.; Supervision, I.K. and E.S.; Project administration, E.S.; Funding acquisition, I.K. and E.S. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

Data is contained within the article or Supplementary Material.

Acknowledgments

We thank A. Manousaki for her technical assistance with the SEM experiments. We thank Klytaimnistra Katsara for her technical assistance with the Raman experiments.

Conflicts of Interest

There are no conflicts to declare.

Footnotes

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Figures and Scheme

Scheme 1. (a) Chemical structure of TPP porphyrin molecule, (b) schematic representation of the single-solvent nucleation and growth protocol for the organization of TPP on different glass substrates and at different temperatures.

Figure 1. SEM images of TPP nanostructures upon evaporation from THF (1 mg/mL) on different glass substrates at 25 °C: (a) on NaPO3 glass, (b) on AgPO3 glass and (c) on SiO2 glass.

Figure 2. SEM images of TPP nanostructures upon evaporation from THF (1 mg/mL) on NaPO3 glass substrates at different temperatures: (a) at 25 °C, (b) at 70 °C, (c) at 170 °C.

Figure 3. Nucleation and growth of TPP upon evaporation from DCM (1 mg/mL) (a) on NaPO3 glass substrate at 25 °C, (b) on SiO2 glass substrate at 25 °C, (c) on NaPO3 glass substrate at 40 °C and (d) on SiO2 glass substrate at 40 °C.

Figure 4. Normalized UV–Vis absorption spectra (a), and fluorescence emission spectra (b), of TPP nanostructures upon evaporation from THF (1 mg/mL) on NaPO3 glass substrate at different temperatures.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/jcs9030116/s1, Figure S1: Nucleation and growth of TPP upon evaporation from DCM (1mg/mL) (a) on Si substrate at 25 °C, (b) on Si substrate at 40 °C, (c) on FTO coated glass substrate at 25 °C, (d) on FTO coated glass substrate at 40 °C; Figure S2: Size histogram of the TPP architectures: (a,b) rectangular sticks on NaPO₃ at 70 °C using THF solvent, (c,d) rectangular sticks of TPP on NaPO₃ at 25 °C using DCM solvent, (e,f) rectangular sticks of TPP on SiO₂ at 25 °C using DCM solvent; Figure S3: (a) UV-Vis absorption spectra and (b) fluorescence emission spectra after excitation at 418 nm, of TPP in THF solution (C = 0.5*10⁻⁶ M); Figure S4: Normalized UV-Vis absorption spectra (a), and fluorescence emission spectra (b), of TPP nanostructures upon evaporation from THF (1mg/mL) on different glass substrates at 25 °C; Figure S5: Normalized UV-Vis absorption spectra (a), and fluorescence emission spectra (b), of TPP nanostructures upon evaporation from DCM (1mg/mL) on NaPO₃ and SiO₂ glass substrates at 25 °C and 40 °C temperatures; Figure S6: Raman spectra of TPP on different glass substrates; Table S1: Absorption and emission peaks of the TPP nanostructures upon evaporation on different glass substrates at various temperatures.

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