Research Article | Open Access
Molecular Dynamics of Film Formation of Metal Tetrasulfonated Phthalocyanine and Poly Amidoamine Dendrimers
We performed molecular dynamics computer simulations to elucidate the behavior and properties of the metal tetrasulfonated phthalocyanine molecule and the poly(amidoamine) dendrimers in self-assembly depositions, respectively, on poly(allylamine hydrochloride) polymer and on film formed by metal tetrasulfonated phthalocyanine with poly(allylamine hydrochloride). Important physical properties of phthalocyanines were obtained such as the kinetic energy and temperature in situ. By the semiempirical model, we also obtained the UV-Vis absorption spectrum of the film formed by cobalt tetrasulfonated phthalocyanine deposited on poly(allylamine hydrochloride). We performed a study with poly(amidoamine) dendrimers on their deposition time on metal tetrasulfonated phthalocyanine, poly(allylamine hydrochloride) film, and we show the relationship of deposition time with the electrical charge and molecular mass of phthalocyanines. The deposition times of the dendrimers, as a function of their mass, were also elucidated.
Studies about the formation of ultrathin organic film [1, 2] have intensified in recent years, since most devices are made with organic materials in the form known as layer by layer, in which two organic molecules are generally used, alternated, in the compound. In the assembly of these alternating layers, the layer by layer (LBL) self-assembly fabrication technique of nanostructured films [3–5] stands out.
In this work, nanostructured ultrathin films are formed based on the LBL technique by physical adsorption, based on the van der Waals interactions and electrostatic interactions between the polyelectrolytes of opposite charges. Computational simulations were performed using molecular dynamics (MD) [6–8] to increase the knowledge about the metal tetrasulfonated phthalocyanine (MTsPc) [9–15] and poly(amidoamine) (PAMAM)dendrimers [16–24] in the deposition process, respectively, on poly(allylamine hydrochloride) (PAH)polymers  and on the film formed by MTsPc with PAH (MTsPc-PAH).
Studies about the deposition of metal phthalocyanines [26–31] have intensified in recent years, due to the prospects of creating new materials  with applications in electronics, for being an organic semiconductor [33, 34], optoelectronics , and in the developing of sensors with greater sensitivity and selectivity of gases and other products [36–40].
Mafatle and Nyokong  have used cobalt phthalocyanine to improve the sensitivity and stability of carbon electrodes for the detection of cresols, chlorophenol, and phenol.
The detection of chlorine gas was performed using a newly developed gas sensor using copper phthalocyanine thin films. Miyata and coworkers  explain that there is an increase of the conductivity of copper phthalocyanine sensors with exposure to chlorine in gas phase.
The PAMAM dendrimer research areas have been diversifying over time. This organic macromolecule, which has a branched and highly symmetric molecular architecture, has revolutionary applications in the medical field as a possible carrier and distributor of specific drugs [43–45]. More recently, PAMAM dendrimers are being used in the development of new organic materials with applications in electronics and optoelectronics, such as the development of sensors, light emitting diodes and more efficient solar cells [46–49].
In the work of Zucolotto and coworkers , biosensors highly sensitive to detection of catechol were obtained through the formation of nanostructured films consisting of alternating layers of Cl-catechol 1,2 dioxygenase and generation G4 PAMAM dendrimer.
We performed molecular dynamics simulations of the self-assembly of MTsPc molecule on the PAH polymer to elucidate its behavior and properties in this process. is the central atom of MTsPc, which varies with the following metallic elements: copper (Cu), cobalt (Co), nickel (Ni), iron (Fe), and manganese (Mn). The MTsPcs are anionic polyelectrolytes and the PAH is the cationic polyelectrolyte. We also obtained the ultraviolet-visible absorption spectrum (UV-Vis) using the semiempirical ZINDO/S method of the organic film CoTsPc-PAH.
Finally, we performed simulations of the self-assembly of PAMAM dendrimers generations GN on the MTsPc-PAH film, where , 2, 3, 4, and 5, showing the behavior of PAMAM dendrimers in this process. These dendrimers are cationic polyelectrolytes.
We obtained the kinetic energy, and temperature in situ, and calculated the molar entropy variation and the ratio between the kinetic energy and temperature in situ of MTsPcs. We also show the deposition time of MTsPcs on PAH, as well as the deposition time of PAMAM dendrimers on the MTsPc-PAH film.
Molecular dynamics methods were used to obtain information about the dynamic behavior of MTsPcs and PAMAM dendrimers in time dependence. The Hamiltonian of the system is given by the following equation: where , , and are the position, momentum, and mass of the particle, respectively, and is the potential energy of the particle.
We defined some physical properties calculated in this work; for example, the molar entropy variation () of this system is given by where is the temperature in situ and is the total energy.
We determined the constant proportionality of the molecules given by where is the kinetic energy of system.
We performed classical molecular dynamics simulations to understand the deposition dynamics of the MTsPc ( = Cu, Co, Ni, Fe, and Mn) on the PAH polymer and PAMAM dendrimers generation GN (, 2, 3, 4, and 5), on the MTsPc-PAH film. The deposition occurs mainly by van der Waals force and electrostatic force between the polyelectrolytes of opposite charge.
For the optimization of the molecular architecture of all the molecules, we used the Polak-Ribiere algorithm (conjugate gradient) with RMS gradient 0.1 kcal/(mol Angstrom). In this paper, we worked with the MM+ force field. The simulations were performed with a step of 1 femtosecond in vacuum.
Optimization energy values of phthalocyanines metal atoms are Mn: −152.4 kcal/mol, Fe: −161.9 kcal/mol, Co: −151.5 kcal/mol, Cu: −131.6 kcal/mol, Ni: −138.5 kcal/mol, and PAH: −1611.1 kcal/mol. Otherwise, the PAMAM dendrimers have optimization energy values of G1: −16.4 kcal/mol, G2: −58.4 kcal/mol, G3: −134.4 kcal/mol, G4: −237.6 kcal/mol, and G5: −327.4 kcal/mol.
3.1. MTsPc-PAH System
After the optimization of the molecules, we put each MTsPc, relaxed, at a distance of about 17 Å from PAH, fixed. We set the initial temperature of the system as 300 K, and the total simulation time was 200 ps, which is enough time for system analysis. We obtained the theoretical UV-Vis absorption spectrum of the nanostructured film formed by the CoTsPc-PAH, by the semi-empirical ZINDO/S method.
3.2. GN PAMAM-MTsPc-PAH System
After the optimizations of the PAMAM dendrimer, each generation, relaxed, was separated from the core, by a distance of 23 Å from the central atom of the film MTsPc-PAH, fixed. The initial temperature of system was 300 K, and total simulation time was 100 ps, which is enough time to equilibrate the system.
4. Results and Discussion
Figure 1 displays the behavior of the MTsPc, where = Cu, Co, Ni, Fe, and Mn, in the deposition process on PAH. From the beginning of the simulation, the MTsPcs begin to move toward PAH due to van der Waals forces and electrostatic forces. We observed an MTsPcs pattern behavior, which occurred between 10 ps and 15 ps of simulation. During this interval, an uprighting of MTsPcs preceding the deposition was verified. This uprighting is due to electrostatic attraction between the sulfonate groups of MTsPcs and the amines of PAH. After the uprighting, the deposition of the MTsPcs occurs. In Figure 2, we show the MTsPc deposition time, which is organized in decreasing order of the atomic mass of the central metal element. It is observed that the CuTsPc, which has the highest molecular mass, takes a longer time to deposit, about 60.40 ps, and the FeTsPc takes a shorter time, about 25.11 ps. It can be seen that the deposition time for MTsPcs tends to decrease with the lower molecular mass. However, in Figure 2, the MnTsPc deposition time was longer than the deposition time of FeTsPc, despite having the lower molecular mass. We can understand this behavior by looking for an electrical charge of the MTsPcs. The FeTsPc region has a greater negative charge than the MnTsPc, which explains their deposition in less time.
Figure 3 illustrates the of MTsPcs versus time. The curves grow over time, due to increased molecular agitation caused by interaction with PAH. The MnTsPc reached the highest value in , about 716 kcal/mol at 200 ps. At the deposition time, there were also peaks in which indicate the MTsPcs deposition.
These peaks are detailed in Figure 4 about FeTsPc. At point , the deposition of FeTsPcs effectively starts. The FeTsPc is attracted by PAH due to van der Waals and electrostatic interaction forces. There is an increase of until point . For point , the intermolecular forces become repulsive due to the great approximation between these polyelectrolytes. The interpolation of the electron clouds between these polyelectrolytes also led to a slowdown of the FeTsPc limiting the approximation with PAH. At point C, there is the closest approach of FeTsPcs with the PAH. The time of point is taken as the deposition time of the FeTsPc on PAH in this case, 24.90 ps. From this point the intermolecular forces become balanced, and FeTsPcs tend to stabilize on the PAH. All other phthalocyanines had these qualitatively similar behaviors.
Figure 5 illustrates the of the MTsPcs versus time. The curve grows in time function, due to increasing atomic agitation. Analyzing the total simulation time, we find that MnTsPc reaches a maximum value of 473 K close to 200 ps.
Figure 6 shows the molar entropy variation as a function of simulation time. At the beginning of the simulation, all MTsPcs have grown ; however, at about 7 ps, a decrease in occurs in the MnTsPc, FeTsPc, CoTsPc, and NiTsPc. They tend to stabilize on the substrate. It can be observed that the of the CuTsPc takes a longer time to start decreasing. At 200 ps of simulation, the CuTsPc has the lowest values, which shows its higher stability. The highest value of occurs with MnTsPc and FeTsPc at 11.45 ps, whose value was 1.30 kcal/mol K.
The ratio of MTsPcs has a constant value equal to 1.50828.
We have shown, in Figure 7, the UV-Vis theoretical absorption spectrum of the CoTsPc, PAH, and the resulting film. Note that the resultant absorption spectrum is formed by contribution of the absorption spectra of CoTsPc and PAH. We also note that at the wavelengths between 268 nm, and 297 nm the absorption of the film is equal to zero; that is, there were no electronic jumps. It could be inferred that the presence of CoTsPc restricts the electronic jump of PAH which in turn also limits the electronic jump of CoTsPc.
Figure 8 illustrates the behavior of the GN PAMAM dendrimers in the deposition process on MTsPc-PAH films. From the beginning of the simulation, the PAMAM begins to move toward MTsPc-PAH due to van der Waals forces and electrostatic forces between the polyelectrolytes. Naturally, the dendrimers are deposited exactly on the MTsPcs films because they are polyelectrolytes with different charges. We found that PAMAM dendrimers are flattened due to the intensity of intermolecular forces. Figure 9 displays the deposition time of GN generations of PAMAM dendrimers on MTsPcs-PAH, where = Fe, Cu, Co, Mn, and Ni. The system that took more time to form was the NiTsPc-G1 PAMAM-PAH, where the G1 PAMAM was deposited at an interval of about 84.5 ps. This result is satisfactory, as the G1 PAMAM has less hydrogen, which has partial positive charges, and the NiTsPc has less negative charge compared with other phthalocyanines. The fastest formed system was the MnTsPc-G5 PAMAM-PAH, at an interval of about 16.3 ps, which is satisfactory, since the G5 PAMAM has more partial positive charges due to hydrogen. The PAMAM generation deposition times tend to decrease with increasing generations, which can be understood by the systematic increase of partial positive charges in these dendrimers.
We found that the CuTsPc takes a longer time to deposit, about 60.40 ps, and that FeTsPc spends a shorter time, about 25.11 ps. Both the molecular mass and the electronic charge have an influence on the deposition of MTsPcs. The deposition time for MTsPcs tends to decrease with reduction of molecular mass. The molecular mass explains the longer deposition time of the phthalocyanine CuTsPc since this has a higher molecular mass. The charge explains the FeTsPc shorter deposition time, since this phthalocyanine has the more negative charge in this work.
The kinetic energy grows with the increase in simulation time, and the in situ temperature and the molar entropy variation decreases with time, since the MTsPcs seek equilibrium as they are deposited on the PAH.
The ratio of the MTsPcs has a constant value equal to 1.50828. We found that the CoTsPc-PAH films have a wavelength range in which the electronic jumps do not occur. This is because this phthalocyanine prevents electronic jumps of the PAH and vice versa. The G1 PAMAM was deposited more slowly on NiTsPc-PAH film at an interval of 84.5 ps, and G5 PAMAM was deposited more quickly on MnTsPc-PAH film at an interval of 16.3 ps. The deposition time of PAMAM dendrimers on the MTsPc-PAH tends to decrease as their generations increase.
L. G. Silva attests to the fact that all authors listed on the title page have read the paper, he attest to the validity and legitimacy of the data and its interpretation, and he agrees to its submission to the Journal of Nanomaterials. The authors are submitting the manuscript as an original paper. The authors listed do not have any financial relation with the commercial identity mentioned in this paper, and there is no conflict of interests.
A. M. J. C. Neto thanks the support given by PIBIC-CNPQ, PROPESP/UFPA, and Reitoria/UFPA. L. G. Silva thanks SIBOP-PROEX/UFPA and PROINT 2012-2013/UFPA.
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